Catalysis

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Catalysis

A range of industrial catalysts in pellet form

An air filter that uses a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature. It can also remove formaldehyde from the air.

Catalysis (/kəˈtæləsɪs/) is the process of change in rate of a chemical reaction by adding a substance known as a catalyst (/ˈkætəlɪst/). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quickly, very small amounts of catalyst often suffice; mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst.

Catalysis may be classified as either homogeneous, whose components are dispersed in the same phase (usually gaseous or liquid) as the reactant, or heterogeneous, whose components are not in the same phase. Enzymes and other biocatalysts are often considered as a third category.

Catalysis is ubiquitous in chemical industry of all kinds. Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture.

The term "catalyst" is derived from Greek καταλύειν, kataluein, meaning "loosen" or "untie". The concept of catalysis was invented by chemist Elizabeth Fulhame, based on her novel work in oxidation-reduction experiments.

General principles

Example

An illustrative example is the effect of catalysts to speed the decomposition of hydrogen peroxide into water and oxygen:

2 H₂O₂ → 2 H₂O + O₂

This reaction proceeds because the reaction products are more stable than the starting compound, but this decomposition is so slow that hydrogen peroxide solutions are commercially available. In the presence of a catalyst such as manganese dioxide this reaction proceeds much more rapidly. This effect is readily seen by the effervescence of oxygen. The catalyst is not consumed in the reaction, and may be recovered unchanged and re-used indefinitely. Accordingly, manganese dioxide is said to catalyze this reaction. In living organisms, this reaction is catalyzed by enzymes (proteins that serve as catalysts) such as catalase.

Units

The SI derived unit for measuring the catalytic activity of a catalyst is the katal, which is quantified in moles per second. The productivity of a catalyst can be described by the turnover number (or TON) and the catalytic activity by the turn over frequency (TOF), which is the TON per time unit. The biochemical equivalent is the enzyme unit. For more information on the efficiency of enzymatic catalysis, see the article on enzymes.

Catalytic reaction mechanisms

Main article: catalytic cycle

In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction mechanism (reaction pathway) having a lower activation energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst usually reacts to form an intermediate, which then regenerates the original catalyst in the process.

As a simple example occurring in the gas phase, the reaction 2 SO₂ + O₂ → 2 SO₃ can be catalyzed by adding nitric oxide. The reaction occurs in two steps:

2 NO + O₂ → 2 NO₂ (rate-determining)

NO₂ + SO₂ → NO + SO₃ (fast)

The NO catalyst is regenerated. The overall rate is the rate of the slow step

v = 2k₁[NO]²[O₂].

An example of heterogeneous catalysis is the reaction of oxygen and hydrogen on the surface of titanium dioxide (TiO₂, or titania) to produce water. Scanning tunneling microscopy showed that the molecules undergo adsorption and dissociation. The dissociated, surface-bound O and H atoms diffuse together. The intermediate reaction states are: HO₂, H₂O₂, then H₃O₂ and the reaction product (water molecule dimers), after which the water molecule desorbs from the catalyst surface.

Reaction energetics

Generic potential energy diagram showing the effect of a catalyst in a hypothetical exothermic chemical reaction X + Y to give Z. The presence of the catalyst opens a different reaction pathway (shown in red) with lower activation energy. The final result and the overall thermodynamics are the same.

Catalysts enable pathways that differ from the uncatalyzed reactions. These pathways have lower activation energy. Consequently, more molecular collisions have the energy needed to reach the transition state. Hence, catalysts can enable reactions that would otherwise be blocked or slowed by a kinetic barrier. The catalyst may increase the reaction rate or selectivity, or enable the reaction at lower temperatures. This effect can be illustrated with an energy profile diagram.

In the catalyzed elementary reaction, catalysts do not change the extent of a reaction: they have no effect on the chemical equilibrium of a reaction. The ratio of the forward and the reverse reaction rates is unaffected (see also thermodynamics). The second law of thermodynamics describes why a catalyst does not change the chemical equilibrium of a reaction. Suppose there was such a catalyst that shifted an equilibrium. Introducing the catalyst to the system would result in a reaction to move to the new equilibrium, producing energy. Production of energy is a necessary result since reactions are spontaneous only if Gibbs free energy is produced, and if there is no energy barrier, there is no need for a catalyst. Then, removing the catalyst would also result in a reaction, producing energy; i.e. the addition and its reverse process, removal, would both produce energy. Thus, a catalyst that could change the equilibrium would be a perpetual motion machine, a contradiction to the laws of thermodynamics. Thus, catalysts do not alter the equilibrium constant. (A catalyst can however change the equilibrium concentrations by reacting in a subsequent step. It is then consumed as the reaction proceeds, and thus it is also a reactant. Illustrative is the base-catalyzed hydrolysis of esters, where the produced carboxylic acid immediately reacts with the base catalyst and thus the reaction equilibrium is shifted towards hydrolysis.)

The catalyst stabilizes the transition state more than it stabilizes the starting material. It decreases the kinetic barrier by decreasing the difference in energy between starting material and the transition state. It does not change the energy difference between starting materials and products (thermodynamic barrier), or the available energy (this is provided by the environment as heat or light).

Related concepts

Some so-called catalysts are really precatalysts. Precatalysts convert to catalysts in the reaction. For example, Wilkinson's catalyst RhCl(PPh₃)₃ loses one triphenylphosphine ligand before entering the true catalytic cycle. Precatalysts are easier to store but are easily activated in situ. Because of this preactivation step, many catalytic reactions involve an induction period.

In cooperative catalysis, chemical species that improve catalytic activity are called cocatalysts or promoters.

In tandem catalysis two or more different catalysts are coupled in a one-pot reaction.

In autocatalysis, the catalyst is a product of the overall reaction, in contrast to all other types of catalysis considered in this article. The simplest example of autocatalysis is a reaction of type A + B → 2 B, in one or in several steps. The overall reaction is just A → B, so that B is a product. But since B is also a reactant, it may be present in the rate equation and affect the reaction rate. As the reaction proceeds, the concentration of B increases and can accelerate the reaction as a catalyst. In effect, the reaction accelerates itself or is autocatalyzed. An example is the hydrolysis of an ester such as aspirin to a carboxylic acid and an alcohol. In the absence of added acid catalysts, the carboxylic acid product catalyzes the hydrolysis.

A true catalyst can work in tandem with a sacrificial catalyst. The true catalyst is consumed in the elementary reaction and turned into a deactivated form. The sacrificial catalyst regenerates the true catalyst for another cycle. The sacrificial catalyst is consumed in the reaction, and as such, it is not really a catalyst, but a reagent. For example, osmium tetroxide (OsO₄) is a good reagent for dihydroxylation, but it is highly toxic and expensive. In Upjohn dihydroxylation, the sacrificial catalyst N-methylmorpholine N-oxide (NMMO) regenerates OsO₄, and only catalytic quantities of OsO₄ are needed.

Classification

Catalysis may be classified as either homogeneous or heterogeneous. A homogeneous catalysis is one whose components are dispersed in the same phase (usually gaseous or liquid) as the reactant's molecules. A heterogeneous catalysis is one where the reaction components are not in the same phase. Enzymes and other biocatalysts are often considered as a third category. Similar mechanistic principles apply to heterogeneous, homogeneous, and biocatalysis.

Heterogeneous catalysis

Main article: Heterogeneous catalysis

The microporous molecular structure of the zeolite ZSM-5 is exploited in catalysts used in refineries

Zeolites are extruded as pellets for easy handling in catalytic reactors.

Heterogeneous catalysts act in a different phase than the reactants. Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture. Important heterogeneous catalysts include zeolites, alumina, higher-order oxides, graphitic carbon, transition metal oxides, metals such as Raney nickel for hydrogenation, and vanadium(V) oxide for oxidation of sulfur dioxide into sulfur trioxide by the contact process.

Diverse mechanisms for reactions on surfaces are known, depending on how the adsorption takes place (Langmuir-Hinshelwood, Eley-Rideal, and Mars-van Krevelen). The total surface area of a solid has an important effect on the reaction rate. The smaller the catalyst particle size, the larger the surface area for a given mass of particles.

A heterogeneous catalyst has active sites, which are the atoms or crystal faces where the reaction actually occurs. Depending on the mechanism, the active site may be either a planar exposed metal surface, a crystal edge with imperfect metal valence, or a complicated combination of the two. Thus, not only most of the volume but also most of the surface of a heterogeneous catalyst may be catalytically inactive. Finding out the nature of the active site requires technically challenging research. Thus, empirical research for finding out new metal combinations for catalysis continues.

For example, in the Haber process, finely divided iron serves as a catalyst for the synthesis of ammonia from nitrogen and hydrogen. The reacting gases adsorb onto active sites on the iron particles. Once physically adsorbed, the reagents undergo chemisorption that results in dissociation into adsorbed atomic species, and new bonds between the resulting fragments form in part due to their closeness. In this way the particularly strong triple bond in nitrogen is broken, which would be extremely uncommon in the gas phase due to its high activation energy. Thus, the activation energy of the overall reaction is lowered, and the rate of reaction increases. Another place where a heterogeneous catalyst is applied is in the oxidation of sulfur dioxide on vanadium(V) oxide for the production of sulfuric acid.

Heterogeneous catalysts are typically "supported," which means that the catalyst is dispersed on a second material that enhances the effectiveness or minimizes its cost. Supports prevent or minimize agglomeration and sintering of small catalyst particles, exposing more surface area, thus catalysts have a higher specific activity (per gram) on support. Sometimes the support is merely a surface on which the catalyst is spread to increase the surface area. More often, the support and the catalyst interact, affecting the catalytic reaction. Supports can also be used in nanoparticle synthesis by providing sites for individual molecules of catalyst to chemically bind. Supports are porous materials with a high surface area, most commonly alumina, zeolites or various kinds of activated carbon. Specialized supports include silicon dioxide, titanium dioxide, calcium carbonate, and barium sulfate.

In slurry reactions, heterogeneous catalysts can be lost by dissolving.

Many heterogeneous catalysts are in fact nanomaterials. Nanomaterial-based catalysts with enzyme-mimicking activities are collectively called as nanozymes.

Electrocatalysts

Main article: Electrocatalyst

In the context of electrochemistry, specifically in fuel cell engineering, various metal-containing catalysts are used to enhance the rates of the half reactions that comprise the fuel cell. One common type of fuel cell electrocatalyst is based upon nanoparticles of platinum that are supported on slightly larger carbon particles. When in contact with one of the electrodes in a fuel cell, this platinum increases the rate of oxygen reduction either to water or to hydroxide or hydrogen peroxide.

Homogeneous catalysis

Main article: Homogeneous catalysis

Homogeneous catalysts function in the same phase as the reactants. Typically homogeneous catalysts are dissolved in a solvent with the substrates. One example of homogeneous catalysis involves the influence of H⁺ on the esterification of carboxylic acids, such as the formation of methyl acetate from acetic acid and methanol. High-volume processes requiring a homogeneous catalyst include hydroformylation, hydrosilylation, hydrocyanation. For inorganic chemists, homogeneous catalysis is often synonymous with organometallic catalysts. Many homogeneous catalysts are however not organometallic, illustrated by the use of cobalt salts that catalyze the oxidation of p-xylene to terephthalic acid.

Organocatalysis

Main article: Organocatalysis

Whereas transition metals sometimes attract most of the attention in the study of catalysis, small organic molecules without metals can also exhibit catalytic properties, as is apparent from the fact that many enzymes lack transition metals. Typically, organic catalysts require a higher loading (amount of catalyst per unit amount of reactant, expressed in mol% amount of substance) than transition metal(-ion)-based catalysts, but these catalysts are usually commercially available in bulk, helping to lower costs. In the early 2000s, these organocatalysts were considered "new generation" and are competitive to traditional metal(-ion)-containing catalysts. Organocatalysts are supposed to operate akin to metal-free enzymes utilizing, e.g., non-covalent interactions such as hydrogen bonding. The discipline organocatalysis is divided into the application of covalent (e.g., proline, DMAP) and non-covalent (e.g., thiourea organocatalysis) organocatalysts referring to the preferred catalyst-substrate binding and interaction, respectively. The Nobel Prize in Chemistry 2021 was awarded jointly to Benjamin List and David W.C. MacMillan "for the development of asymmetric organocatalysis."

Photocatalysts

Main article: Photocatalysis

Photocatalysis is the phenomenon where the catalyst can receive light to generate an excited state that effect redox reactions. Singlet oxygen is usually produced by photocatalysis. Photocatalysts are components of dye-sensitized solar cells.

Enzymes and biocatalysts

Main article: enzyme catalysis

In biology, enzymes are protein-based catalysts in metabolism and catabolism. Most biocatalysts are enzymes, but other non-protein-based classes of biomolecules also exhibit catalytic properties including ribozymes, and synthetic deoxyribozymes.

Biocatalysts can be thought of as an intermediate between homogeneous and heterogeneous catalysts, although strictly speaking soluble enzymes are homogeneous catalysts and membrane-bound enzymes are heterogeneous. Several factors affect the activity of enzymes (and other catalysts) including temperature, pH, the concentration of enzymes, substrate, and products. A particularly important reagent in enzymatic reactions is water, which is the product of many bond-forming reactions and a reactant in many bond-breaking processes.

In biocatalysis, enzymes are employed to prepare many commodity chemicals including high-fructose corn syrup and acrylamide.

Some monoclonal antibodies whose binding target is a stable molecule that resembles the transition state of a chemical reaction can function as weak catalysts for that chemical reaction by lowering its activation energy. Such catalytic antibodies are sometimes called "abzymes".

Significance

Left: Partially caramelized cube sugar, Right: burning cube sugar with ash as catalyst

Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture. In 2005, catalytic processes generated about $900 billion in products worldwide. Catalysis is so pervasive that subareas are not readily classified. Some areas of particular concentration are surveyed below.

Energy processing

Petroleum refining makes intensive use of catalysis for alkylation, catalytic cracking (breaking long-chain hydrocarbons into smaller pieces), naphtha reforming and steam reforming (conversion of hydrocarbons into synthesis gas). Even the exhaust from the burning of fossil fuels is treated via catalysis: Catalytic converters, typically composed of platinum and rhodium, break down some of the more harmful byproducts of automobile exhaust.

2 CO + 2 NO → 2 CO₂ + N₂

With regard to synthetic fuels, an old but still important process is the Fischer-Tropsch synthesis of hydrocarbons from synthesis gas, which itself is processed via water-gas shift reactions, catalyzed by iron. The Sabatier reaction produces methane from carbon dioxide and hydrogen. Biodiesel and related biofuels require processing via both inorganic and biocatalysts.

Fuel cells rely on catalysts for both the anodic and cathodic reactions.

Catalytic heaters generate flameless heat from a supply of combustible fuel.

Bulk chemicals

Typical vanadium pentoxide catalyst used in sulfuric acid production for an intermediate reaction to convert sulfur dioxide to sulfur trioxide.

Some of the largest-scale chemicals are produced via catalytic oxidation, often using oxygen. Examples include nitric acid (from ammonia), sulfuric acid (from sulfur dioxide to sulfur trioxide by the contact process), terephthalic acid from p-xylene, acrylic acid from propylene or propane and acrylonitrile from propane and ammonia.

The production of ammonia is one of the largest-scale and most energy-intensive processes. In the Haber process nitrogen is combined with hydrogen over an iron oxide catalyst. Methanol is prepared from carbon monoxide or carbon dioxide but using copper-zinc catalysts.

Bulk polymers derived from ethylene and propylene are often prepared via Ziegler-Natta catalysis. Polyesters, polyamides, and isocyanates are derived via acid-base catalysis.

Most carbonylation processes require metal catalysts, examples include the Monsanto acetic acid process and hydroformylation.

Fine chemicals

Many fine chemicals are prepared via catalysis; methods include those of heavy industry as well as more specialized processes that would be prohibitively expensive on a large scale. Examples include the Heck reaction, and Friedel–Crafts reactions. Because most bioactive compounds are chiral, many pharmaceuticals are produced by enantioselective catalysis (catalytic asymmetric synthesis). (R)-1,2-Propandiol, the precursor to the antibacterial levofloxacin, can be synthesized efficiently from hydroxyacetone by using catalysts based on BINAP-ruthenium complexes, in Noyori asymmetric hydrogenation:

Food processing

One of the most obvious applications of catalysis is the hydrogenation (reaction with hydrogen gas) of fats using nickel catalyst to produce margarine. Many other foodstuffs are prepared via biocatalysis (see below).

Environment

Catalysis affects the environment by increasing the efficiency of industrial processes, but catalysis also plays a direct role in the environment. A notable example is the catalytic role of chlorine free radicals in the breakdown of ozone. These radicals are formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs).

Cl^· + O₃ → ClO^· + O₂

ClO^· + O^· → Cl^· + O₂

History

The term "catalyst", broadly defined as anything that increases the rate of a process, is derived from Greek καταλύειν, meaning "to annul," or "to untie," or "to pick up". The concept of catalysis was invented by chemist Elizabeth Fulhame and described in a 1794 book, based on her novel work in oxidation–reduction reactions. The first chemical reaction in organic chemistry that knowingly used a catalyst was studied in 1811 by Gottlieb Kirchhoff, who discovered the acid-catalyzed conversion of starch to glucose. The term catalysis was later used by Jöns Jakob Berzelius in 1835 to describe reactions that are accelerated by substances that remain unchanged after the reaction. Fulhame, who predated Berzelius, did work with water as opposed to metals in her reduction experiments. Other 18th century chemists who worked in catalysis were Eilhard Mitscherlich who referred to it as contact processes, and Johann Wolfgang Döbereiner who spoke of contact action. He developed Döbereiner's lamp, a lighter based on hydrogen and a platinum sponge, which became a commercial success in the 1820s that lives on today. Humphry Davy discovered the use of platinum in catalysis. In the 1880s, Wilhelm Ostwald at Leipzig University started a systematic investigation into reactions that were catalyzed by the presence of acids and bases, and found that chemical reactions occur at finite rates and that these rates can be used to determine the strengths of acids and bases. For this work, Ostwald was awarded the 1909 Nobel Prize in Chemistry. Vladimir Ipatieff performed some of the earliest industrial scale reactions, including the discovery and commercialization of oligomerization and the development of catalysts for hydrogenation.

Inhibitors, poisons, and promoters

An added substance that lowers the rate is called a reaction inhibitor if reversible and catalyst poisons if irreversible. Promoters are substances that increase the catalytic activity, even though they are not catalysts by themselves.

Inhibitors are sometimes referred to as "negative catalysts" since they decrease the reaction rate. However the term inhibitor is preferred since they do not work by introducing a reaction path with higher activation energy; this would not lower the rate since the reaction would continue to occur by the non-catalyzed path. Instead, they act either by deactivating catalysts or by removing reaction intermediates such as free radicals. In heterogeneous catalysis, coking inhibits the catalyst, which becomes covered by polymeric side products.

The inhibitor may modify selectivity in addition to rate. For instance, in the hydrogenation of alkynes to alkenes, a palladium (Pd) catalyst partly "poisoned" with lead(II) acetate (Pb(CH₃CO₂)₂) can be used. Without the deactivation of the catalyst, the alkene produced would be further hydrogenated to alkane.

The inhibitor can produce this effect by, e.g., selectively poisoning only certain types of active sites. Another mechanism is the modification of surface geometry. For instance, in hydrogenation operations, large planes of metal surface function as sites of hydrogenolysis catalysis while sites catalyzing hydrogenation of unsaturates are smaller. Thus, a poison that covers the surface randomly will tend to lower the number of uncontaminated large planes but leave proportionally smaller sites free, thus changing the hydrogenation vs. hydrogenolysis selectivity. Many other mechanisms are also possible.

Promoters can cover up the surface to prevent the production of a mat of coke, or even actively remove such material (e.g., rhenium on platinum in platforming). They can aid the dispersion of the catalytic material or bind to reagents.

Trigonometric functions

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Trigonometric_functions

 

Trigonometry

Basis of trigonometry: if two right triangles have equal acute angles, they are similar, so their side lengths are proportional.

In mathematics, the trigonometric functions (also called circular functions, angle functions or goniometric functions) are real functions which relate an angle of a right-angled triangle to ratios of two side lengths. They are widely used in all sciences that are related to geometry, such as navigation, solid mechanics, celestial mechanics, geodesy, and many others. They are among the simplest periodic functions, and as such are also widely used for studying periodic phenomena through Fourier analysis.

The trigonometric functions most widely used in modern mathematics are the sine, the cosine, and the tangent. Their reciprocals are respectively the cosecant, the secant, and the cotangent, which are less used. Each of these six trigonometric functions has a corresponding inverse function, and an analog among the hyperbolic functions.

The oldest definitions of trigonometric functions, related to right-angle triangles, define them only for acute angles. To extend the sine and cosine functions to functions whose domain is the whole real line, geometrical definitions using the standard unit circle (i.e., a circle with radius 1 unit) are often used; then the domain of the other functions is the real line with some isolated points removed. Modern definitions express trigonometric functions as infinite series or as solutions of differential equations. This allows extending the domain of sine and cosine functions to the whole complex plane, and the domain of the other trigonometric functions to the complex plane with some isolated points removed.

Notation

Conventionally, an abbreviation of each trigonometric function's name is used as its symbol in formulas. Today, the most common versions of these abbreviations are "sin" for sine, "cos" for cosine, "tan" or "tg" for tangent, "sec" for secant, "csc" or "cosec" for cosecant, and "cot" or "ctg" for cotangent. Historically, these abbreviations were first used in prose sentences to indicate particular line segments or their lengths related to an arc of an arbitrary circle, and later to indicate ratios of lengths, but as the function concept developed in the 17th–18th century, they began to be considered as functions of real-number-valued angle measures, and written with functional notation, for example sin(x). Parentheses are still often omitted to reduce clutter, but are sometimes necessary; for example the expression ${\displaystyle \sin x+y}$ would typically be interpreted to mean ${\displaystyle \sin (x)+y,}$ so parentheses are required to express ${\displaystyle \sin (x+y).}$

A positive integer appearing as a superscript after the symbol of the function denotes exponentiation, not function composition. For example ${\displaystyle {\sin }^{2}x}$ and ${\displaystyle {\sin }^2(x)}$ denote ${\displaystyle \sin (x)\cdot \sin (x),}$ not ${\displaystyle \sin (\sin x).}$ This differs from the (historically later) general functional notation in which ${\displaystyle f^2(x)=(f\circ f)(x)=f(f(x)).}$

However, the exponent ${\displaystyle -1}$ is commonly used to denote the inverse function, not the reciprocal. For example ${\displaystyle {\sin }^{-1}x}$ and ${\displaystyle {\sin }^{-1}(x)}$ denote the inverse trigonometric function alternatively written ${\displaystyle \arcsin x:}$ The equation ${\displaystyle \theta ={\sin }^{-1}x}$ implies ${\displaystyle \sin \theta =x,}$ not ${\displaystyle \theta \cdot \sin x=1.}$ In this case, the superscript could be considered as denoting a composed or iterated function, but negative superscripts other than ${\displaystyle -1}$ are not in common use.

Right-angled triangle definitions

In this right triangle, denoting the measure of angle BAC as A: sin A = a/c; cos A = b/c; tan A = a/b.

Plot of the six trigonometric functions, the unit circle, and a line for the angle θ = 0.7 radians. The points labelled 1, Sec(θ), Csc(θ) represent the length of the line segment from the origin to that point. Sin(θ), Tan(θ), and 1 are the heights to the line starting from the x-axis, while Cos(θ), 1, and Cot(θ) are lengths along the x-axis starting from the origin.

If the acute angle θ is given, then any right triangles that have an angle of θ are similar to each other. This means that the ratio of any two side lengths depends only on θ. Thus these six ratios define six functions of θ, which are the trigonometric functions. In the following definitions, the hypotenuse is the length of the side opposite the right angle, opposite represents the side opposite the given angle θ, and adjacent represents the side between the angle θ and the right angle.

sine ${\displaystyle \sin \theta =\frac{\mathrm{o}\mathrm{p}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{e}}{\mathrm{h}\mathrm{y}\mathrm{p}\mathrm{o}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{u}\mathrm{s}\mathrm{e}}}$	cosecant ${\displaystyle \csc \theta =\frac{\mathrm{h}\mathrm{y}\mathrm{p}\mathrm{o}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{u}\mathrm{s}\mathrm{e}}{\mathrm{o}\mathrm{p}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{e}}}$
cosine ${\displaystyle \cos \theta =\frac{\mathrm{a}\mathrm{d}\mathrm{j}\mathrm{a}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}}{\mathrm{h}\mathrm{y}\mathrm{p}\mathrm{o}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{u}\mathrm{s}\mathrm{e}}}$	secant ${\displaystyle \sec \theta =\frac{\mathrm{h}\mathrm{y}\mathrm{p}\mathrm{o}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{u}\mathrm{s}\mathrm{e}}{\mathrm{a}\mathrm{d}\mathrm{j}\mathrm{a}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}}}$
tangent ${\displaystyle \tan \theta =\frac{\mathrm{o}\mathrm{p}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{e}}{\mathrm{a}\mathrm{d}\mathrm{j}\mathrm{a}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}}}$	cotangent ${\displaystyle \cot \theta =\frac{\mathrm{a}\mathrm{d}\mathrm{j}\mathrm{a}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}}{\mathrm{o}\mathrm{p}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{e}}}$

In a right-angled triangle, the sum of the two acute angles is a right angle, that is, 90° or π/2 radians. Therefore ${\displaystyle \sin (\theta )}$ and ${\displaystyle \cos ({90}^{\circ }-\theta )}$ represent the same ratio, and thus are equal. This identity and analogous relationships between the other trigonometric functions are summarized in the following table.

Top: Trigonometric function sin θ for selected angles θ, π − θ, π + θ, and 2π − θ in the four quadrants.
Bottom: Graph of sine function versus angle. Angles from the top panel are identified.

Summary of relationships between trigonometric functions

Function	Description	Relationship
		using radians	using degrees
sine	opposite/hypotenuse	${\displaystyle \sin \theta =\cos \left(\frac{\pi }{2}-\theta \right)=\frac{1}{\csc \theta }}$	${\displaystyle \sin x=\cos \left({90}^{\circ }-x\right)=\frac{1}{\csc x}}$
cosine	adjacent/hypotenuse	${\displaystyle \cos \theta =\sin \left(\frac{\pi }{2}-\theta \right)=\frac{1}{\sec \theta }}$	${\displaystyle \cos x=\sin \left({90}^{\circ }-x\right)=\frac{1}{\sec x}}$
tangent	opposite/adjacent	${\displaystyle \tan \theta =\frac{\sin \theta }{\cos \theta }=\cot \left(\frac{\pi }{2}-\theta \right)=\frac{1}{\cot \theta }}$	${\displaystyle \tan x=\frac{\sin x}{\cos x}=\cot \left({90}^{\circ }-x\right)=\frac{1}{\cot x}}$
cotangent	adjacent/opposite	${\displaystyle \cot \theta =\frac{\cos \theta }{\sin \theta }=\tan \left(\frac{\pi }{2}-\theta \right)=\frac{1}{\tan \theta }}$	${\displaystyle \cot x=\frac{\cos x}{\sin x}=\tan \left({90}^{\circ }-x\right)=\frac{1}{\tan x}}$
secant	hypotenuse/adjacent	${\displaystyle \sec \theta =\csc \left(\frac{\pi }{2}-\theta \right)=\frac{1}{\cos \theta }}$	${\displaystyle \sec x=\csc \left({90}^{\circ }-x\right)=\frac{1}{\cos x}}$
cosecant	hypotenuse/opposite	${\displaystyle \csc \theta =\sec \left(\frac{\pi }{2}-\theta \right)=\frac{1}{\sin \theta }}$	${\displaystyle \csc x=\sec \left({90}^{\circ }-x\right)=\frac{1}{\sin x}}$

Radians versus degrees

In geometric applications, the argument of a trigonometric function is generally the measure of an angle. For this purpose, any angular unit is convenient. One common unit is degrees, in which a right angle is 90° and a complete turn is 360° (particularly in elementary mathematics).

However, in calculus and mathematical analysis, the trigonometric functions are generally regarded more abstractly as functions of real or complex numbers, rather than angles. In fact, the functions sin and cos can be defined for all complex numbers in terms of the exponential function, via power series, or as solutions to differential equations given particular initial values (see below), without reference to any geometric notions. The other four trigonometric functions (tan, cot, sec, csc) can be defined as quotients and reciprocals of sin and cos, except where zero occurs in the denominator. It can be proved, for real arguments, that these definitions coincide with elementary geometric definitions if the argument is regarded as an angle given in radians. Moreover, these definitions result in simple expressions for the derivatives and indefinite integrals for the trigonometric functions. Thus, in settings beyond elementary geometry, radians are regarded as the mathematically natural unit for describing angle measures.

When radians (rad) are employed, the angle is given as the length of the arc of the unit circle subtended by it: the angle that subtends an arc of length 1 on the unit circle is 1 rad (≈ 57.3°), and a complete turn (360°) is an angle of 2π (≈ 6.28) rad. For real number x, the notations sin x, cos x, etc. refer to the value of the trigonometric functions evaluated at an angle of x rad. If units of degrees are intended, the degree sign must be explicitly shown (e.g., sin x°, cos x°, etc.). Using this standard notation, the argument x for the trigonometric functions satisfies the relationship x = (180x/π)°, so that, for example, sin π = sin 180° when we take x = π. In this way, the degree symbol can be regarded as a mathematical constant such that 1° = π/180 ≈ 0.0175.

Unit-circle definitions

In this illustration, the six trigonometric functions of an arbitrary angle θ are represented as Cartesian coordinates of points related to the unit circle. The ordinates of A, B and D are sin θ, tan θ and csc θ, respectively, while the abscissas of A, C and E are cos θ, cot θ and sec θ, respectively.

Signs of trigonometric functions in each quadrant. The mnemonic "all science teachers (are) crazy" lists the functions which are positive from quadrants I to IV. This is a variation on the mnemonic "All Students Take Calculus".

The six trigonometric functions can be defined as coordinate values of points on the Euclidean plane that are related to the unit circle, which is the circle of radius one centered at the origin O of this coordinate system. While right-angled triangle definitions allow for the definition of the trigonometric functions for angles between 0 and $\frac{\pi }{2}$ radians (90°), the unit circle definitions allow the domain of trigonometric functions to be extended to all positive and negative real numbers.

Let ${\displaystyle \mathcal{L}}$ be the ray obtained by rotating by an angle θ the positive half of the x-axis (counterclockwise rotation for ${\displaystyle \theta >0,}$ and clockwise rotation for ${\displaystyle \theta <0}$). This ray intersects the unit circle at the point ${\displaystyle \mathrm{A}=(x_{\mathrm{A}},y_{\mathrm{A}}).}$ The ray ${\displaystyle \mathcal{L},}$ extended to a line if necessary, intersects the line of equation ${\displaystyle x=1}$ at point ${\displaystyle \mathrm{B}=(1,y_{\mathrm{B}}),}$ and the line of equation ${\displaystyle y=1}$ at point ${\displaystyle \mathrm{C}=(x_{\mathrm{C}},1).}$ The tangent line to the unit circle at the point A, is perpendicular to ${\displaystyle \mathcal{L},}$ and intersects the y- and x-axes at points ${\displaystyle \mathrm{D}=(0,y_{\mathrm{D}})}$ and ${\displaystyle \mathrm{E}=(x_{\mathrm{E}},0).}$ The coordinates of these points give the values of all trigonometric functions for any arbitrary real value of θ in the following manner.

The trigonometric functions cos and sin are defined, respectively, as the x- and y-coordinate values of point A. That is,

${\displaystyle \cos \theta =x_{\mathrm{A}}}$ and ${\displaystyle \sin \theta =y_{\mathrm{A}}.}$

In the range ${\displaystyle 0\le \theta \le \pi /2}$, this definition coincides with the right-angled triangle definition, by taking the right-angled triangle to have the unit radius OA as hypotenuse. And since the equation ${\displaystyle x^2+y^2=1}$ holds for all points ${\displaystyle \mathrm{P}=(x,y)}$ on the unit circle, this definition of cosine and sine also satisfies the Pythagorean identity.

${\displaystyle {\cos }^2\theta +{\sin }^2\theta =1.}$

The other trigonometric functions can be found along the unit circle as

${\displaystyle \tan \theta =y_{\mathrm{B}}}$ and ${\displaystyle \cot \theta =x_{\mathrm{C}},}$

${\displaystyle \csc \theta =y_{\mathrm{D}}}$ and ${\displaystyle \sec \theta =x_{\mathrm{E}}.}$

By applying the Pythagorean identity and geometric proof methods, these definitions can readily be shown to coincide with the definitions of tangent, cotangent, secant and cosecant in terms of sine and cosine, that is

${\displaystyle \tan \theta =\frac{\sin \theta }{\cos \theta },\cot \theta =\frac{\cos \theta }{\sin \theta },\sec \theta =\frac{1}{\cos \theta },\csc \theta =\frac{1}{\sin \theta }.}$

Trigonometric functions: Sine, Cosine, Tangent, Cosecant (dotted), Secant (dotted), Cotangent (dotted) – animation

Since a rotation of an angle of ${\displaystyle \pm 2\pi }$ does not change the position or size of a shape, the points A, B, C, D, and E are the same for two angles whose difference is an integer multiple of ${\displaystyle 2\pi }$. Thus trigonometric functions are periodic functions with period ${\displaystyle 2\pi }$. That is, the equalities

${\displaystyle \sin \theta =\sin \left(\theta +2k\pi \right)}$ and ${\displaystyle \cos \theta =\cos \left(\theta +2k\pi \right)}$

hold for any angle θ and any integer k. The same is true for the four other trigonometric functions. By observing the sign and the monotonicity of the functions sine, cosine, cosecant, and secant in the four quadrants, one can show that ${\displaystyle 2\pi }$ is the smallest value for which they are periodic (i.e., ${\displaystyle 2\pi }$ is the fundamental period of these functions). However, after a rotation by an angle ${\displaystyle \pi }$, the points B and C already return to their original position, so that the tangent function and the cotangent function have a fundamental period of ${\displaystyle \pi }$. That is, the equalities

${\displaystyle \tan \theta =\tan (\theta +k\pi )}$ and ${\displaystyle \cot \theta =\cot (\theta +k\pi )}$

hold for any angle θ and any integer k.

Algebraic values

The unit circle, with some points labeled with their cosine and sine (in this order), and the corresponding angles in radians and degrees.

The algebraic expressions for the most important angles are as follows:

${\displaystyle \sin 0=\sin 0^{\circ }=\frac{\sqrt{0}}{2}=0}$ (zero angle)

${\displaystyle \sin \frac{\pi }{6}=\sin {30}^{\circ }=\frac{\sqrt{1}}{2}=\frac{1}{2}}$

${\displaystyle \sin \frac{\pi }{4}=\sin {45}^{\circ }=\frac{\sqrt{2}}{2}=\frac{1}{\sqrt{2}}}$

${\displaystyle \sin \frac{\pi }{3}=\sin {60}^{\circ }=\frac{\sqrt{3}}{2}}$

${\displaystyle \sin \frac{\pi }{2}=\sin {90}^{\circ }=\frac{\sqrt{4}}{2}=1}$ (right angle)

Writing the numerators as square roots of consecutive non-negative integers, with a denominator of 2, provides an easy way to remember the values.

Such simple expressions generally do not exist for other angles which are rational multiples of a right angle.

• For an angle which, measured in degrees, is a multiple of three, the exact trigonometric values of the sine and the cosine may be expressed in terms of square roots. These values of the sine and the cosine may thus be constructed by ruler and compass.
• For an angle of an integer number of degrees, the sine and the cosine may be expressed in terms of square roots and the cube root of a non-real complex number. Galois theory allows a proof that, if the angle is not a multiple of 3°, non-real cube roots are unavoidable.
• For an angle which, expressed in degrees, is a rational number, the sine and the cosine are algebraic numbers, which may be expressed in terms of nth roots. This results from the fact that the Galois groups of the cyclotomic polynomials are cyclic.
• For an angle which, expressed in degrees, is not a rational number, then either the angle or both the sine and the cosine are transcendental numbers. This is a corollary of Baker's theorem, proved in 1966.

Simple algebraic values

Main article: Exact trigonometric values § Common angles

The following table lists the sines, cosines, and tangents of multiples of 15 degrees from 0 to 90 degrees.

Angle, θ, in		${\displaystyle \sin (\theta )}$	${\displaystyle \cos (\theta )}$	${\displaystyle \tan (\theta )}$
radians	degrees
${\displaystyle 0}$	${\displaystyle 0^{\circ }}$	${\displaystyle 0}$	${\displaystyle 1}$	${\displaystyle 0}$
${\displaystyle \frac{\pi }{12}}$	${\displaystyle {15}^{\circ }}$	${\displaystyle \frac{\sqrt{6}-\sqrt{2}}{4}}$	${\displaystyle \frac{\sqrt{6}+\sqrt{2}}{4}}$	${\displaystyle 2-\sqrt{3}}$
${\displaystyle \frac{\pi }{6}}$	${\displaystyle {30}^{\circ }}$	${\displaystyle \frac{1}{2}}$	${\displaystyle \frac{\sqrt{3}}{2}}$	${\displaystyle \frac{\sqrt{3}}{3}}$
${\displaystyle \frac{\pi }{4}}$	${\displaystyle {45}^{\circ }}$	${\displaystyle \frac{\sqrt{2}}{2}}$	${\displaystyle \frac{\sqrt{2}}{2}}$	${\displaystyle 1}$
${\displaystyle \frac{\pi }{3}}$	${\displaystyle {60}^{\circ }}$	${\displaystyle \frac{\sqrt{3}}{2}}$	${\displaystyle \frac{1}{2}}$	${\displaystyle \sqrt{3}}$
${\displaystyle \frac{5\pi }{12}}$	${\displaystyle {75}^{\circ }}$	${\displaystyle \frac{\sqrt{6}+\sqrt{2}}{4}}$	${\displaystyle \frac{\sqrt{6}-\sqrt{2}}{4}}$	${\displaystyle 2+\sqrt{3}}$
${\displaystyle \frac{\pi }{2}}$	${\displaystyle {90}^{\circ }}$	${\displaystyle 1}$	${\displaystyle 0}$	Undefined

In calculus

Graphs of sine, cosine and tangent

 

The sine function (blue) is closely approximated by its Taylor polynomial of degree 7 (pink) for a full cycle centered on the origin.

Animation for the approximation of cosine via Taylor polynomials.

${\displaystyle \cos (x)}$ together with the first Taylor polynomials ${\displaystyle p_n(x)=\sum _{k=0}^n(-1{)}^k\frac{x^{2k}}{(2k)!}}$

The modern trend in mathematics is to build geometry from calculus rather than the converse. Therefore, except at a very elementary level, trigonometric functions are defined using the methods of calculus.

Trigonometric functions are differentiable and analytic at every point where they are defined; that is, everywhere for the sine and the cosine, and, for the tangent, everywhere except at π/2 + kπ for every integer k.

The trigonometric function are periodic functions, and their primitive period is 2π for the sine and the cosine, and π for the tangent, which is increasing in each open interval (π/2 + kπ, π/2 + (k + 1)π). At each end point of these intervals, the tangent function has a vertical asymptote.

In calculus, there are two equivalent definitions of trigonometric functions, either using power series or differential equations. These definitions are equivalent, as starting from one of them, it is easy to retrieve the other as a property. However the definition through differential equations is somehow more natural, since, for example, the choice of the coefficients of the power series may appear as quite arbitrary, and the Pythagorean identity is much easier to deduce from the differential equations.

Definition by differential equations

Sine and cosine can be defined as the unique solution to the initial value problem:

${\displaystyle \frac{d}{dx}\sin x=\cos x,\frac{d}{dx}\cos x=-\sin x,\sin (0)=0,\cos (0)=1.}$

Differentiating again, $\frac{d^2}{d{x}^2}\sin x=\frac{d}{dx}\cos x=-\sin x$ and $\frac{d^2}{d{x}^2}\cos x=-\frac{d}{dx}\sin x=-\cos x$, so both sine and cosine are solutions of the ordinary differential equation

${\displaystyle y^{″}+y=0.}$

Applying the quotient rule to the tangent ${\displaystyle \tan x=\sin x/\cos x}$, we derive

${\displaystyle \frac{d}{dx}\tan x=\frac{{\cos }^{2}x+{\sin }^{2}x}{{\cos }^{2}x}=1+{\tan }^{2}x={\sec }^{2}x.}$

Power series expansion

Applying the differential equations to power series with indeterminate coefficients, one may deduce recurrence relations for the coefficients of the Taylor series of the sine and cosine functions. These recurrence relations are easy to solve, and give the series expansions^[12]

${\displaystyle \begin{array}{rl}\sin x& =x-\frac{x^3}{3!}+\frac{x^5}{5!}-\frac{x^7}{7!}+\cdots \\ & =\sum _{n=0}^{\mathrm{\infty }}\frac{(-1{)}^n}{(2n+1)!}{x}^{2n+1}\\ \cos x& =1-\frac{x^2}{2!}+\frac{x^4}{4!}-\frac{x^6}{6!}+\cdots \\ & =\sum _{n=0}^{\mathrm{\infty }}\frac{(-1{)}^n}{(2n)!}{x}^{2n}.\end{array}}$

The radius of convergence of these series is infinite. Therefore, the sine and the cosine can be extended to entire functions (also called "sine" and "cosine"), which are (by definition) complex-valued functions that are defined and holomorphic on the whole complex plane.

Being defined as fractions of entire functions, the other trigonometric functions may be extended to meromorphic functions, that is functions that are holomorphic in the whole complex plane, except some isolated points called poles. Here, the poles are the numbers of the form $(2k+1)\frac{\pi }{2}$ for the tangent and the secant, or ${\displaystyle k\pi }$ for the cotangent and the cosecant, where k is an arbitrary integer.

Recurrences relations may also be computed for the coefficients of the Taylor series of the other trigonometric functions. These series have a finite radius of convergence. Their coefficients have a combinatorial interpretation: they enumerate alternating permutations of finite sets.

More precisely, defining

U_n, the nth up/down number,

B_n, the nth Bernoulli number, and

E_n, is the nth Euler number,

one has the following series expansions:

${\displaystyle \begin{array}{rl}\tan x& =\sum _{n=0}^{\mathrm{\infty }}\frac{U_{2n+1}}{(2n+1)!}{x}^{2n+1}\\ & =\sum _{n=1}^{\mathrm{\infty }}\frac{(-1{)}^{n-1}{2}^{2n}\left(2^{2n}-1\right)B_{2n}}{(2n)!}{x}^{2n-1}\\ & =x+\frac{1}{3}{x}^3+\frac{2}{15}{x}^5+\frac{17}{315}{x}^7+\cdots ,\text{for }|x|<\frac{\pi }{2}.\end{array}}$

${\displaystyle \begin{array}{rl}\csc x& =\sum _{n=0}^{\mathrm{\infty }}\frac{(-1{)}^{n+1}2\left(2^{2n-1}-1\right)B_{2n}}{(2n)!}{x}^{2n-1}\\ & =x^{-1}+\frac{1}{6}x+\frac{7}{360}{x}^3+\frac{31}{15120}{x}^5+\cdots ,\text{for }0<|x|<\pi .\end{array}}$

${\displaystyle \begin{array}{rl}\sec x& =\sum _{n=0}^{\mathrm{\infty }}\frac{U_{2n}}{(2n)!}{x}^{2n}=\sum _{n=0}^{\mathrm{\infty }}\frac{(-1{)}^n{E}_{2n}}{(2n)!}{x}^{2n}\\ & =1+\frac{1}{2}{x}^2+\frac{5}{24}{x}^4+\frac{61}{720}{x}^6+\cdots ,\text{for }|x|<\frac{\pi }{2}.\end{array}}$

${\displaystyle \begin{array}{rl}\cot x& =\sum _{n=0}^{\mathrm{\infty }}\frac{(-1{)}^n{2}^{2n}{B}_{2n}}{(2n)!}{x}^{2n-1}\\ & =x^{-1}-\frac{1}{3}x-\frac{1}{45}{x}^3-\frac{2}{945}{x}^5-\cdots ,\text{for }0<|x|<\pi .\end{array}}$

Continued fraction expansion

The following expansions are valid in the whole complex plane:

${\displaystyle \sin x=\frac{x}{1+\frac{x^2}{2\cdot 3-x^2+\frac{2\cdot 3x^2}{4\cdot 5-x^2+\frac{4\cdot 5x^2}{6\cdot 7-x^2+\ddots }}}}}$

${\displaystyle \cos x=\frac{1}{1+\frac{x^2}{1\cdot 2-x^2+\frac{1\cdot 2x^2}{3\cdot 4-x^2+\frac{3\cdot 4x^2}{5\cdot 6-x^2+\ddots }}}}}$

${\displaystyle \tan x=\frac{x}{1-\frac{x^2}{3-\frac{x^2}{5-\frac{x^2}{7-\ddots }}}}=\frac{1}{\frac{1}{x}-\frac{1}{\frac{3}{x}-\frac{1}{\frac{5}{x}-\frac{1}{\frac{7}{x}-\ddots }}}}}$

The last one was used in the historically first proof that π is irrational.

Partial fraction expansion

There is a series representation as partial fraction expansion where just translated reciprocal functions are summed up, such that the poles of the cotangent function and the reciprocal functions match:

${\displaystyle \pi \cot \pi x=\underset{N\to \mathrm{\infty }}{lim}\sum _{n=-N}^N\frac{1}{x+n}.}$

This identity can be proved with the Herglotz trick. Combining the (–n)th with the nth term lead to absolutely convergent series:

${\displaystyle \pi \cot \pi x=\frac{1}{x}+2x\sum _{n=1}^{\mathrm{\infty }}\frac{1}{x^2-n^2}.}$

Similarly, one can find a partial fraction expansion for the secant, cosecant and tangent functions:

${\displaystyle \pi \csc \pi x=\sum _{n=-\mathrm{\infty }}^{\mathrm{\infty }}\frac{(-1{)}^n}{x+n}=\frac{1}{x}+2x\sum _{n=1}^{\mathrm{\infty }}\frac{(-1{)}^n}{x^2-n^2},}$

${\displaystyle {\pi }^2{\csc }^2\pi x=\sum _{n=-\mathrm{\infty }}^{\mathrm{\infty }}\frac{1}{(x+n{)}^2},}$

${\displaystyle \pi \sec \pi x=\sum _{n=0}^{\mathrm{\infty }}(-1{)}^n\frac{(2n+1)}{(n+\frac{1}{2}{)}^2-x^2},}$

${\displaystyle \pi \tan \pi x=2x\sum _{n=0}^{\mathrm{\infty }}\frac{1}{(n+\frac{1}{2}{)}^2-x^2}.}$

Infinite product expansion

The following infinite product for the sine is of great importance in complex analysis:

${\displaystyle \sin z=z\prod _{n=1}^{\mathrm{\infty }}\left(1-\frac{z^2}{n^2{\pi }^2}\right),z\in \mathbb{C}.}$

For the proof of this expansion, see Sine. From this, it can be deduced that

${\displaystyle \cos z=\prod _{n=1}^{\mathrm{\infty }}\left(1-\frac{z^2}{(n-1/2{)}^2{\pi }^2}\right),z\in \mathbb{C}.}$

Relationship to exponential function (Euler's formula)

${\displaystyle \cos (\theta )}$ and ${\displaystyle \sin (\theta )}$ are the real and imaginary part of ${\displaystyle e^{i\theta }}$ respectively.

Euler's formula relates sine and cosine to the exponential function:

${\displaystyle e^{ix}=\cos x+i\sin x.}$

This formula is commonly considered for real values of x, but it remains true for all complex values.

Proof: Let ${\displaystyle f_1(x)=\cos x+i\sin x,}$ and ${\displaystyle f_2(x)=e^{ix}.}$ One has ${\displaystyle d{f}_j(x)/dx=i{f}_j(x)}$ for j = 1, 2. The quotient rule implies thus that ${\displaystyle d/dx(f_1(x)/f_2(x))=0}$. Therefore, ${\displaystyle f_1(x)/f_2(x)}$ is a constant function, which equals 1, as ${\displaystyle f_1(0)=f_2(0)=1.}$ This proves the formula.

One has

${\displaystyle \begin{array}{rl}{e}^{ix}& =\cos x+i\sin x\\ e^{-ix}& =\cos x-i\sin x.\end{array}}$

Solving this linear system in sine and cosine, one can express them in terms of the exponential function:

${\displaystyle \begin{array}{rl}\sin x& =\frac{e^{ix}-e^{-ix}}{2i}\\ \cos x& =\frac{e^{ix}+e^{-ix}}{2}.\end{array}}$

When x is real, this may be rewritten as

${\displaystyle \cos x=\mathrm{Re}\left(e^{ix}\right),\sin x=\mathrm{Im}\left(e^{ix}\right).}$

Most trigonometric identities can be proved by expressing trigonometric functions in terms of the complex exponential function by using above formulas, and then using the identity ${\displaystyle e^{a+b}=e^a{e}^b}$ for simplifying the result.

Definitions using functional equations

One can also define the trigonometric functions using various functional equations.

For example, the sine and the cosine form the unique pair of continuous functions that satisfy the difference formula

${\displaystyle \cos (x-y)=\cos x\cos y+\sin x\sin y}$

and the added condition

${\displaystyle 0<x\cos x<\sin x<x\text{ for }0<x<1.}$

In the complex plane

The sine and cosine of a complex number ${\displaystyle z=x+iy}$ can be expressed in terms of real sines, cosines, and hyperbolic functions as follows:

${\displaystyle \begin{array}{rl}\sin z& =\sin x\cosh y+i\cos x\sinh y\\ \cos z& =\cos x\cosh y-i\sin x\sinh y\end{array}}$

By taking advantage of domain coloring, it is possible to graph the trigonometric functions as complex-valued functions. Various features unique to the complex functions can be seen from the graph; for example, the sine and cosine functions can be seen to be unbounded as the imaginary part of ${\displaystyle z}$ becomes larger (since the color white represents infinity), and the fact that the functions contain simple zeros or poles is apparent from the fact that the hue cycles around each zero or pole exactly once. Comparing these graphs with those of the corresponding Hyperbolic functions highlights the relationships between the two.

Trigonometric functions in the complex plane

${\displaystyle \sin z}$ ${\displaystyle \cos z}$

${\displaystyle \tan z}$ ${\displaystyle \cot z}$

${\displaystyle \sec z}$ ${\displaystyle \csc z}$

Basic identities

Many identities interrelate the trigonometric functions. This section contains the most basic ones; for more identities, see List of trigonometric identities. These identities may be proved geometrically from the unit-circle definitions or the right-angled-triangle definitions (although, for the latter definitions, care must be taken for angles that are not in the interval [0, π/2], see Proofs of trigonometric identities). For non-geometrical proofs using only tools of calculus, one may use directly the differential equations, in a way that is similar to that of the above proof of Euler's identity. One can also use Euler's identity for expressing all trigonometric functions in terms of complex exponentials and using properties of the exponential function.

Parity

The cosine and the secant are even functions; the other trigonometric functions are odd functions. That is:

${\displaystyle \begin{array}{rl}\sin (-x)& =-\sin x\\ \cos (-x)& =\cos x\\ \tan (-x)& =-\tan x\\ \cot (-x)& =-\cot x\\ \csc (-x)& =-\csc x\\ \sec (-x)& =\sec x.\end{array}}$

Periods

All trigonometric functions are periodic functions of period 2π. This is the smallest period, except for the tangent and the cotangent, which have π as smallest period. This means that, for every integer k, one has

${\displaystyle \begin{array}{lrl}\sin (x+& 2k\pi )& =\sin x\\ \cos (x+& 2k\pi )& =\cos x\\ \tan (x+& k\pi )& =\tan x\\ \cot (x+& k\pi )& =\cot x\\ \csc (x+& 2k\pi )& =\csc x\\ \sec (x+& 2k\pi )& =\sec x.\end{array}}$

Pythagorean identity

The Pythagorean identity, is the expression of the Pythagorean theorem in terms of trigonometric functions. It is

${\displaystyle {\sin }^{2}x+{\cos }^{2}x=1}$.

Dividing through by either ${\displaystyle {\cos }^{2}x}$ or ${\displaystyle {\sin }^{2}x}$ gives

${\displaystyle {\tan }^{2}x+1={\sec }^{2}x}$

and

${\displaystyle 1+{\cot }^{2}x={\csc }^{2}x}$.

Sum and difference formulas

The sum and difference formulas allow expanding the sine, the cosine, and the tangent of a sum or a difference of two angles in terms of sines and cosines and tangents of the angles themselves. These can be derived geometrically, using arguments that date to Ptolemy. One can also produce them algebraically using Euler's formula.

Sum

${\displaystyle \begin{array}{rl}\sin \left(x+y\right)& =\sin x\cos y+\cos x\sin y,\\ \cos \left(x+y\right)& =\cos x\cos y-\sin x\sin y,\\ \tan (x+y)& =\frac{\tan x+\tan y}{1-\tan x\tan y}.\end{array}}$

Difference

${\displaystyle \begin{array}{rl}\sin \left(x-y\right)& =\sin x\cos y-\cos x\sin y,\\ \cos \left(x-y\right)& =\cos x\cos y+\sin x\sin y,\\ \tan (x-y)& =\frac{\tan x-\tan y}{1+\tan x\tan y}.\end{array}}$

When the two angles are equal, the sum formulas reduce to simpler equations known as the double-angle formulae.

${\displaystyle \begin{array}{rl}\sin 2x& =2\sin x\cos x=\frac{2\tan x}{1+{\tan }^{2}x},\\ \cos 2x& ={\cos }^{2}x-{\sin }^{2}x=2{\cos }^{2}x-1=1-2{\sin }^{2}x=\frac{1-{\tan }^{2}x}{1+{\tan }^{2}x},\\ \tan 2x& =\frac{2\tan x}{1-{\tan }^{2}x}.\end{array}}$

These identities can be used to derive the product-to-sum identities.

By setting ${\displaystyle t=\tan \frac{1}{2}\theta ,}$ all trigonometric functions of ${\displaystyle \theta }$ can be expressed as rational fractions of ${\displaystyle t}$:

${\displaystyle \begin{array}{rl}\sin \theta & =\frac{2t}{1+t^2},\\ \cos \theta & =\frac{1-t^2}{1+t^2},\\ \tan \theta & =\frac{2t}{1-t^2}.\end{array}}$

Together with

${\displaystyle d\theta =\frac{2}{1+t^2}dt,}$

this is the tangent half-angle substitution, which reduces the computation of integrals and antiderivatives of trigonometric functions to that of rational fractions.

Derivatives and antiderivatives

The derivatives of trigonometric functions result from those of sine and cosine by applying quotient rule. The values given for the antiderivatives in the following table can be verified by differentiating them. The number C is a constant of integration.

${\displaystyle f(x)}$	${\displaystyle f^{\prime }(x)}$	$\int f(x)dx$
${\displaystyle \sin x}$	${\displaystyle \cos x}$	${\displaystyle -\cos x+C}$
${\displaystyle \cos x}$	${\displaystyle -\sin x}$	${\displaystyle \sin x+C}$
${\displaystyle \tan x}$	${\displaystyle {\sec }^{2}x}$	${\displaystyle \ln \left|\sec x\right|+C}$
${\displaystyle \csc x}$	${\displaystyle -\csc x\cot x}$	${\displaystyle \ln \left|\csc x-\cot x\right|+C}$
${\displaystyle \sec x}$	${\displaystyle \sec x\tan x}$	${\displaystyle \ln \left|\sec x+\tan x\right|+C}$
${\displaystyle \cot x}$	${\displaystyle -{\csc }^{2}x}$	${\displaystyle \ln \left|\sin x\right|+C}$

Note: For ${\displaystyle 0<x<\pi }$ the integral of ${\displaystyle \csc x}$ can also be written as ${\displaystyle -\mathrm{arsinh}(\cot x),}$ and for the integral of ${\displaystyle \sec x}$ for ${\displaystyle -\pi /2<x<\pi /2}$ as ${\displaystyle \mathrm{arsinh}(\tan x),}$ where ${\displaystyle \mathrm{arsinh}}$ is the inverse hyperbolic sine.

Alternatively, the derivatives of the 'co-functions' can be obtained using trigonometric identities and the chain rule:

${\displaystyle \begin{array}{rl}\frac{d\cos x}{dx}& =\frac{d}{dx}\sin (\pi /2-x)=-\cos (\pi /2-x)=-\sin x,\\ \frac{d\csc x}{dx}& =\frac{d}{dx}\sec (\pi /2-x)=-\sec (\pi /2-x)\tan (\pi /2-x)=-\csc x\cot x,\\ \frac{d\cot x}{dx}& =\frac{d}{dx}\tan (\pi /2-x)=-{\sec }^2(\pi /2-x)=-{\csc }^{2}x.\end{array}}$

Inverse functions

Main article: Inverse trigonometric functions

The trigonometric functions are periodic, and hence not injective, so strictly speaking, they do not have an inverse function. However, on each interval on which a trigonometric function is monotonic, one can define an inverse function, and this defines inverse trigonometric functions as multivalued functions. To define a true inverse function, one must restrict the domain to an interval where the function is monotonic, and is thus bijective from this interval to its image by the function. The common choice for this interval, called the set of principal values, is given in the following table. As usual, the inverse trigonometric functions are denoted with the prefix "arc" before the name or its abbreviation of the function.

Function	Definition	Domain	Set of principal values
${\displaystyle y=\arcsin x}$	${\displaystyle \sin y=x}$	${\displaystyle -1\le x\le 1}$	$-\frac{\pi }{2}\le y\le \frac{\pi }{2}$
${\displaystyle y=\arccos x}$	${\displaystyle \cos y=x}$	${\displaystyle -1\le x\le 1}$	$0\le y\le \pi$
${\displaystyle y=\arctan x}$	${\displaystyle \tan y=x}$	${\displaystyle -\mathrm{\infty }<x<\mathrm{\infty }}$	$-\frac{\pi }{2}<y<\frac{\pi }{2}$
${\displaystyle y=\mathrm{arccot}x}$	${\displaystyle \cot y=x}$	${\displaystyle -\mathrm{\infty }<x<\mathrm{\infty }}$	$0<y<\pi$
${\displaystyle y=\mathrm{arcsec}x}$	${\displaystyle \sec y=x}$	${\displaystyle x<-1\text{ or }x>1}$	$0\le y\le \pi ,y\ne \frac{\pi }{2}$
${\displaystyle y=\mathrm{arccsc}x}$	${\displaystyle \csc y=x}$	${\displaystyle x<-1\text{ or }x>1}$	$-\frac{\pi }{2}\le y\le \frac{\pi }{2},y\ne 0$

The notations sin⁻¹, cos⁻¹, etc. are often used for arcsin and arccos, etc. When this notation is used, inverse functions could be confused with multiplicative inverses. The notation with the "arc" prefix avoids such a confusion, though "arcsec" for arcsecant can be confused with "arcsecond".

Just like the sine and cosine, the inverse trigonometric functions can also be expressed in terms of infinite series. They can also be expressed in terms of complex logarithms.

Applications

Main article: Uses of trigonometry

Angles and sides of a triangle

In this section A, B, C denote the three (interior) angles of a triangle, and a, b, c denote the lengths of the respective opposite edges. They are related by various formulas, which are named by the trigonometric functions they involve.

Law of sines

Main article: Law of sines

The law of sines states that for an arbitrary triangle with sides a, b, and c and angles opposite those sides A, B and C:

$${\displaystyle \frac{\sin A}{a}=\frac{\sin B}{b}=\frac{\sin C}{c}=\frac{2\mathrm{\Delta }}{abc},}$$

where Δ is the area of the triangle, or, equivalently,

$${\displaystyle \frac{a}{\sin A}=\frac{b}{\sin B}=\frac{c}{\sin C}=2R,}$$

where R is the triangle's circumradius.

It can be proved by dividing the triangle into two right ones and using the above definition of sine. The law of sines is useful for computing the lengths of the unknown sides in a triangle if two angles and one side are known. This is a common situation occurring in triangulation, a technique to determine unknown distances by measuring two angles and an accessible enclosed distance.

Law of cosines

Main article: Law of cosines

The law of cosines (also known as the cosine formula or cosine rule) is an extension of the Pythagorean theorem:

$${\displaystyle c^2=a^2+b^2-2ab\cos C,}$$

or equivalently,

$${\displaystyle \cos C=\frac{a^2+b^2-c^2}{2ab}.}$$

In this formula the angle at C is opposite to the side c. This theorem can be proved by dividing the triangle into two right ones and using the Pythagorean theorem.

The law of cosines can be used to determine a side of a triangle if two sides and the angle between them are known. It can also be used to find the cosines of an angle (and consequently the angles themselves) if the lengths of all the sides are known.

Law of tangents

Main article: Law of tangents

The law of tangents says that:

${\displaystyle \frac{\tan \frac{A-B}{2}}{\tan \frac{A+B}{2}}=\frac{a-b}{a+b}}$.

Law of cotangents

Main article: Law of cotangents

If s is the triangle's semiperimeter, (a + b + c)/2, and r is the radius of the triangle's incircle, then rs is the triangle's area. Therefore Heron's formula implies that:

${\displaystyle r=\sqrt{\frac{1}{s}(s-a)(s-b)(s-c)}}$.

The law of cotangents says that:

${\displaystyle \cot \frac{A}{2}=\frac{s-a}{r}}$

It follows that

${\displaystyle \frac{\cot {\displaystyle \frac{A}{2}}}{s-a}=\frac{\cot {\displaystyle \frac{B}{2}}}{s-b}=\frac{\cot {\displaystyle \frac{C}{2}}}{s-c}=\frac{1}{r}.}$

Periodic functions

A Lissajous curve, a figure formed with a trigonometry-based function.

An animation of the additive synthesis of a square wave with an increasing number of harmonics

Sinusoidal basis functions (bottom) can form a sawtooth wave (top) when added. All the basis functions have nodes at the nodes of the sawtooth, and all but the fundamental (k = 1) have additional nodes. The oscillation seen about the sawtooth when k is large is called the Gibbs phenomenon

The trigonometric functions are also important in physics. The sine and the cosine functions, for example, are used to describe simple harmonic motion, which models many natural phenomena, such as the movement of a mass attached to a spring and, for small angles, the pendular motion of a mass hanging by a string. The sine and cosine functions are one-dimensional projections of uniform circular motion.

Trigonometric functions also prove to be useful in the study of general periodic functions. The characteristic wave patterns of periodic functions are useful for modeling recurring phenomena such as sound or light waves.

Under rather general conditions, a periodic function f (x) can be expressed as a sum of sine waves or cosine waves in a Fourier series. Denoting the sine or cosine basis functions by φ_k, the expansion of the periodic function f (t) takes the form:

$${\displaystyle f(t)=\sum _{k=1}^{\mathrm{\infty }}{c}_k{\phi }_k(t).}$$

For example, the square wave can be written as the Fourier series

$${\displaystyle f_{\text{square}}(t)=\frac{4}{\pi }\sum _{k=1}^{\mathrm{\infty }}\frac{\sin ((2k-1)t)}{2k-1}.}$$

In the animation of a square wave at top right it can be seen that just a few terms already produce a fairly good approximation. The superposition of several terms in the expansion of a sawtooth wave are shown underneath.

History

Main article: History of trigonometry

While the early study of trigonometry can be traced to antiquity, the trigonometric functions as they are in use today were developed in the medieval period. The chord function was discovered by Hipparchus of Nicaea (180–125 BCE) and Ptolemy of Roman Egypt (90–165 CE). The functions of sine and versine (1 – cosine) can be traced back to the jyā and koti-jyā functions used in Gupta period Indian astronomy (Aryabhatiya, Surya Siddhanta), via translation from Sanskrit to Arabic and then from Arabic to Latin. (See Aryabhata's sine table.)

All six trigonometric functions in current use were known in Islamic mathematics by the 9th century, as was the law of sines, used in solving triangles. With the exception of the sine (which was adopted from Indian mathematics), the other five modern trigonometric functions were discovered by Persian and Arab mathematicians, including the cosine, tangent, cotangent, secant and cosecant. Al-Khwārizmī (c. 780–850) produced tables of sines, cosines and tangents. Circa 830, Habash al-Hasib al-Marwazi discovered the cotangent, and produced tables of tangents and cotangents. Muhammad ibn Jābir al-Harrānī al-Battānī (853–929) discovered the reciprocal functions of secant and cosecant, and produced the first table of cosecants for each degree from 1° to 90°. The trigonometric functions were later studied by mathematicians including Omar Khayyám, Bhāskara II, Nasir al-Din al-Tusi, Jamshīd al-Kāshī (14th century), Ulugh Beg (14th century), Regiomontanus (1464), Rheticus, and Rheticus' student Valentinus Otho.

Madhava of Sangamagrama (c. 1400) made early strides in the analysis of trigonometric functions in terms of infinite series. (See Madhava series and Madhava's sine table.)

The tangent function was brought to Europe by Giovanni Bianchini in 1467 in trigonometry tables he created to support the calculation of stellar coordinates.

The terms tangent and secant were first introduced by the Danish mathematician Thomas Fincke in his book Geometria rotundi (1583).

The 17th century French mathematician Albert Girard made the first published use of the abbreviations sin, cos, and tan in his book Trigonométrie.

In a paper published in 1682, Gottfried Leibniz proved that sin x is not an algebraic function of x. Though introduced as ratios of sides of a right triangle, and thus appearing to be rational functions, Leibnitz result established that they are actually transcendental functions of their argument. The task of assimilating circular functions into algebraic expressions was accomplished by Euler in his Introduction to the Analysis of the Infinite (1748). His method was to show that the sine and cosine functions are alternating series formed from the even and odd terms respectively of the exponential series. He presented "Euler's formula", as well as near-modern abbreviations (sin., cos., tang., cot., sec., and cosec.).

A few functions were common historically, but are now seldom used, such as the chord, the versine (which appeared in the earliest tables), the coversine, the haversine, the exsecant and the excosecant. The list of trigonometric identities shows more relations between these functions.

• crd(θ) = 2 sin(θ/2)
• versin(θ) = 1 − cos(θ) = 2 sin²(θ/2)
• coversin(θ) = 1 − sin(θ) = versin(π/2 − θ)
• haversin(θ) = 1/2versin(θ) = sin²(θ/2)
• exsec(θ) = sec(θ) − 1
• excsc(θ) = exsec(π/2 − θ) = csc(θ) − 1

Etymology

Main article: History of trigonometry § Etymology

The word sine derives from Latin sinus, meaning "bend; bay", and more specifically "the hanging fold of the upper part of a toga", "the bosom of a garment", which was chosen as the translation of what was interpreted as the Arabic word jaib, meaning "pocket" or "fold" in the twelfth-century translations of works by Al-Battani and al-Khwārizmī into Medieval Latin. The choice was based on a misreading of the Arabic written form j-y-b (جيب), which itself originated as a transliteration from Sanskrit jīvā, which along with its synonym jyā (the standard Sanskrit term for the sine) translates to "bowstring", being in turn adopted from Ancient Greek χορδή "string".

The word tangent comes from Latin tangens meaning "touching", since the line touches the circle of unit radius, whereas secant stems from Latin secans—"cutting"—since the line cuts the circle.

The prefix "co-" (in "cosine", "cotangent", "cosecant") is found in Edmund Gunter's Canon triangulorum (1620), which defines the cosinus as an abbreviation for the sinus complementi (sine of the complementary angle) and proceeds to define the cotangens similarly.