Trigonometric functions

76 languages

• Article
• Talk

• Read
• Edit
• View history

From Wikipedia, the free encyclopedia

Trigonometry
Outline History Usage Functions (inverse) Generalized trigonometry
Reference
Identities Exact constants Tables Unit circle
Laws and theorems
Sines Cosines Tangents Cotangents Pythagorean theorem
Calculus
Trigonometric substitution Integrals (inverse functions) Derivatives
v t e

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^[1][2]) 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[edit]

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 ${\displaystyle \sin �+�}$ would typically be interpreted to mean ${\displaystyle \sin (�)+�,}$ so parentheses are required to express ${\displaystyle \sin (�+�).}$

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

However, the exponent ${\displaystyle -1}$ is commonly used to denote the inverse function, not the reciprocal. For example ${\displaystyle {\sin }^{-1}�}$ and ${\displaystyle {\sin }^{-1}(�)}$ denote the inverse trigonometric function alternatively written ${\displaystyle \arcsin �:}$ The equation ${\displaystyle �={\sin }^{-1}�}$ implies ${\displaystyle \sin �=�,}$ not ${\displaystyle �\cdot \sin �=1.}$ 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[edit]

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.^[3][4]

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

In a right-angled triangle, the sum of the two acute angles is a right angle, that is, 90° or π/2 radians. Therefore ${\displaystyle \sin (�)}$ and ${\displaystyle \cos ({90}^{\circ }-�)}$ 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^[5]

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

Radians versus degrees[edit]

In geometric applications, the argument of a trigonometric function is generally the measure of an angle. For this purpose, any angular unit is convenient, and angles are most commonly measured in conventional units of 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^[6] or as solutions to differential equations given particular initial values^[7] (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.^[6] Moreover, these definitions result in simple expressions for the derivatives and indefinite integrals for the trigonometric functions.^[8] 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[edit]

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.^[9] 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 $\frac{�}{2}$ radians (90°), the unit circle definitions allow the domain of trigonometric functions to be extended to all positive and negative real numbers.

Let ${\displaystyle \mathcal{�}}$ be the ray obtained by rotating by an angle θ the positive half of the x-axis (counterclockwise rotation for  and clockwise rotation for ). This ray intersects the unit circle at the point ${\displaystyle \mathrm{A}=({�}_{\mathrm{A}},{�}_{\mathrm{A}}).}$ The ray ${\displaystyle \mathcal{�},}$ extended to a line if necessary, intersects the line of equation ${\displaystyle �=1}$ at point ${\displaystyle \mathrm{B}=(1,{�}_{\mathrm{B}}),}$ and the line of equation ${\displaystyle �=1}$ at point ${\displaystyle \mathrm{C}=({�}_{\mathrm{C}},1).}$ The tangent line to the unit circle at the point A, is perpendicular to ${\displaystyle \mathcal{�},}$ and intersects the y- and x-axes at points ${\displaystyle \mathrm{D}=(0,{�}_{\mathrm{D}})}$ and ${\displaystyle \mathrm{E}=({�}_{\mathrm{E}},0).}$ 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,

${\displaystyle \cos �={�}_{\mathrm{A}}}$ and ${\displaystyle \sin �={�}_{\mathrm{A}}.}$^[10]

In the range ${\displaystyle 0\le �\le �/2}$, 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 ${\displaystyle {�}^2+{�}^2=1}$ holds for all points ${\displaystyle \mathrm{P}=(�,�)}$ on the unit circle, this definition of cosine and sine also satisfies the Pythagorean identity.

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

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

${\displaystyle \tan �={�}_{\mathrm{B}}}$ and ${\displaystyle \cot �={�}_{\mathrm{C}},}$

${\displaystyle \csc �={�}_{\mathrm{D}}}$ and ${\displaystyle \sec �={�}_{\mathrm{E}}.}$

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

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

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

Since a rotation of an angle of ${\displaystyle \pm 2�}$ 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 ${\displaystyle 2�}$. Thus trigonometric functions are periodic functions with period ${\displaystyle 2�}$. That is, the equalities

${\displaystyle \sin �=\sin \left(�+2��\right)}$ and ${\displaystyle \cos �=\cos \left(�+2��\right)}$

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 ${\displaystyle 2�}$ is the smallest value for which they are periodic (i.e., ${\displaystyle 2�}$ is the fundamental period of these functions). However, after a rotation by an angle ${\displaystyle �}$, 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 ${\displaystyle �}$. That is, the equalities

${\displaystyle \tan �=\tan (�+��)}$ and ${\displaystyle \cot �=\cot (�+��)}$

hold for any angle θ and any integer k.