In mathematics, trigonometric identities are equalities that involve trigonometric functions and are true for every value of the occurring variables for which both sides of the equality are defined. Geometrically, these are identities involving certain functions of one or more angles. They are distinct from triangle identities, which are identities potentially involving angles but also involving side lengths or other lengths of a triangle.
These identities are useful whenever expressions involving trigonometric functions need to be simplified. An important application is the integration of non-trigonometric functions: a common technique involves first using the substitution rule with a trigonometric function, and then simplifying the resulting integral with a trigonometric identity.
This article uses Greek letters such as alpha (), beta (), gamma (), and theta () to represent angles. Several different units of angle measure are widely used, including degree, radian, and gradian (gons):
- 1 complete rotation (turn)
If not specifically annotated by () for degree or () for gradian, all values for angles in this article are assumed to be given in radian.
The following table shows for some common angles their conversions and the values of the basic trigonometric functions:
Results for other angles can be found at Trigonometric constants expressed in real radicals. Per Niven's theorem, are the only rational numbers that, taken in degrees, result in a rational sine-value for the corresponding angle within the first turn, which may account for their popularity in examples. The analogous condition for the unit radian requires that the argument divided by is rational, and yields the solutions
The functions sine, cosine and tangent of an angle are sometimes referred to as the primary or basic trigonometric functions. Their usual abbreviations are and respectively, where denotes the angle. The parentheses around the argument of the functions are often omitted, e.g., and if an interpretation is unambiguously possible.
The sine of an angle is defined, in the context of a right triangle, as the ratio of the length of the side that is opposite to the angle divided by the length of the longest side of the triangle (the hypotenuse).
The cosine of an angle in this context is the ratio of the length of the side that is adjacent to the angle divided by the length of the hypotenuse.
The tangent of an angle in this context is the ratio of the length of the side that is opposite to the angle divided by the length of the side that is adjacent to the angle. This is the same as the ratio of the sine to the cosine of this angle, as can be seen by substituting the definitions of and from above:
The remaining trigonometric functions secant (), cosecant (), and cotangent () are defined as the reciprocal functions of cosine, sine, and tangent, respectively. Rarely, these are called the secondary trigonometric functions:
These definitions are sometimes referred to as ratio identities.
indicates the sign function, which is defined as:
The inverse trigonometric functions are partial inverse functions for the trigonometric functions. For example, the inverse function for the sine, known as the inverse sine () or arcsine (arcsin or asin), satisfies
This article will denote the inverse of a trigonometric function by prefixing its name with "". The notation is shown in the table below.
|Range of usual|
principal values of inverse
Similarly, the domain of and is the set
These inverse trigonometric functions are related to one another by the formulas:
Solutions to elementary trigonometric equations
The following table shows how inverse trigonometric functions may be used to solve equalities involving the six standard trigonometric functions. It is assumed that the given values and all lie within appropriate ranges so that the relevant expressions below are well-defined. In the table below, "for some " is just another way of saying "for some integer "
|Equation||if and only if||Solution||where...|
For example, if then for some While if then for some where is even if ; odd if The equations and have the same solutions as and respectively. In all equations above except for those just solved (i.e. except for / and /), for fixed and the integer in the solution's formula is uniquely determined by
The table below shows how two angles and must be related if their values under a given trigonometric function are equal or negatives of each other.
|Equation||if and only if||Solution||where...||Also a solution to|
In trigonometry, the basic relationship between the sine and the cosine is given by the Pythagorean identity:
where the sign depends on the quadrant of
Dividing this identity by either or yields the other two Pythagorean identities:
Using these identities together with the ratio identities, it is possible to express any trigonometric function in terms of any other (up to a plus or minus sign):
|in terms of|
The versine, coversine, haversine, and exsecant were used in navigation. For example, the haversine formula was used to calculate the distance between two points on a sphere. They are rarely used today.
|(right) complementary angle, co-angle|
|versed sine, versine|
|versed cosine, vercosine|
|coversed sine, coversine|
|coversed cosine, covercosine|
|half versed sine, haversine|
|half versed cosine, havercosine|
|half coversed sine, hacoversine
|half coversed cosine, hacovercosine
|exterior secant, exsecant|
|exterior cosecant, excosecant|
Reflections, shifts, and periodicity
By examining the unit circle, one can establish the following properties of the trigonometric functions.
When the direction of a Euclidean vector is represented by an angle this is the angle determined by the free vector (starting at the origin) and the positive -unit vector. The same concept may also be applied to lines in a Euclidean space, where the angle is that determined by a parallel to the given line through the origin and the positive -axis. If a line (vector) with direction is reflected about a line with direction then the direction angle of this reflected line (vector) has the value
The values of the trigonometric functions of these angles for specific angles satisfy simple identities: either they are equal, or have opposite signs, or employ the complementary trigonometric function. These are also known as reduction formulae.
| reflected in 
|reflected in||reflected in||reflected in|| reflected in |
Shifts and periodicity
Through shifting the arguments of trigonometric functions by certain angles, changing the sign or applying complementary trigonometric functions can sometimes express particular results more simply. Some examples of shifts are shown below in the table.
- A full turn, or or radian leaves the unit circle fixed and is the smallest interval for which the trigonometric functions sin, cos, sec, and csc repeat their values and is thus their period. Shifting arguments of any periodic function by any integer multiple of a full period preserves the function value of the unshifted argument.
- A half turn, or or radian is the period of and as can be seen from these definitions and the period of the defining trigonometric functions. Therefore, shifting the arguments of and by any multiple of does not change their function values.
- For the functions sin, cos, sec, and csc with period half a turn is half their period. For this shift, they change the sign of their values, as can be seen from the unit circle again. This new value repeats after any additional shift of so all together they change the sign for a shift by any odd multiple of that is, by with k an arbitrary integer. Any even multiple of is of course just a full period, and a backward shift by half a period is the same as a backward shift by one full period plus one shift forward by half a period.
- A quarter turn, or or radian is a half-period shift for and with period (), yielding the function value of applying the complementary function to the unshifted argument. By the argument above this also holds for a shift by any odd multiple of the half period.
- For the four other trigonometric functions, a quarter turn also represents a quarter period. A shift by an arbitrary multiple of a quarter period that is not covered by a multiple of half periods can be decomposed in an integer multiple of periods, plus or minus one quarter period. The terms expressing these multiples are The forward/backward shifts by one quarter period are reflected in the table below. Again, these shifts yield function values, employing the respective complementary function applied to the unshifted argument.
- Shifting the arguments of and by their quarter period () does not yield such simple results.
|Shift by one quarter period||Shift by one half period||Shift by full periods||Period|
Angle sum and difference identities
These are also known as the angle addition and subtraction theorems (or formulae). The identities can be derived by combining right triangles such as in the adjacent diagram, or by considering the invariance of the length of a chord on a unit circle given a particular central angle. The most intuitive derivation uses rotation matrices (see below).
For acute angles and whose sum is non-obtuse, a concise diagram (shown) illustrates the angle sum formulae for sine and cosine: The bold segment labeled "1" has unit length and serves as the hypotenuse of a right triangle with angle ; the opposite and adjacent legs for this angle have respective lengths and The leg is itself the hypotenuse of a right triangle with angle ; that triangle's legs, therefore, have lengths given by and multiplied by The leg, as hypotenuse of another right triangle with angle likewise leads to segments of length and Now, we observe that the "1" segment is also the hypotenuse of a right triangle with angle ; the leg opposite this angle necessarily has length while the leg adjacent has length Consequently, as the opposing sides of the diagram's outer rectangle are equal, we deduce
Relocating one of the named angles yields a variant of the diagram that demonstrates the angle difference formulae for sine and cosine. (The diagram admits further variants to accommodate angles and sums greater than a right angle.) Dividing all elements of the diagram by provides yet another variant (shown) illustrating the angle sum formula for tangent.
These identities have applications in, for example, in-phase and quadrature components.
The sum and difference formulae for sine and cosine follow from the fact that a rotation of the plane by angle following a rotation by is equal to a rotation by In terms of rotation matrices:
The matrix inverse for a rotation is the rotation with the negative of the angle
which is also the matrix transpose.
These formulae show that these matrices form a representation of the rotation group in the plane (technically, the special orthogonal group SO(2)), since the composition law is fulfilled and inverses exist. Furthermore, matrix multiplication of the rotation matrix for an angle with a column vector will rotate the column vector counterclockwise by the angle
Since multiplication by a complex number of unit length rotates the complex plane by the argument of the number, the above multiplication of rotation matrices is equivalent to a multiplication of complex numbers:
Sines and cosines of sums of infinitely many angles
When the series converges absolutely then
Because the series converges absolutely, it is necessarily the case that and In particular, in these two identities an asymmetry appears that is not seen in the case of sums of finitely many angles: in each product, there are only finitely many sine factors but there are cofinitely many cosine factors. Terms with infinitely many sine factors would necessarily be equal to zero.
When only finitely many of the angles are nonzero then only finitely many of the terms on the right side are nonzero because all but finitely many sine factors vanish. Furthermore, in each term all but finitely many of the cosine factors are unity.
Tangents and cotangents of sums
Let (for ) be the kth-degree elementary symmetric polynomial in the variables
using the sine and cosine sum formulae above.
The number of terms on the right side depends on the number of terms on the left side.
Secants and cosecants of sums
where is the kth-degree elementary symmetric polynomial in the n variables and the number of terms in the denominator and the number of factors in the product in the numerator depend on the number of terms in the sum on the left. The case of only finitely many terms can be proved by mathematical induction on the number of such terms.
|Tn is the nth Chebyshev polynomial|||
|de Moivre's formula, i is the imaginary unit|||
Double-angle, triple-angle, and half-angle formulae
Formulae for twice an angle.
Formulae for triple angles.
These can be shown by using either the sum and difference identities or the multiple-angle formulae.
The fact that the triple-angle formula for sine and cosine only involves powers of a single function allows one to relate the geometric problem of a compass and straightedge construction of angle trisection to the algebraic problem of solving a cubic equation, which allows one to prove that trisection is in general impossible using the given tools, by field theory.
A formula for computing the trigonometric identities for the one-third angle exists, but it requires finding the zeroes of the cubic equation 4x3 − 3x + d = 0, where is the value of the cosine function at the one-third angle and d is the known value of the cosine function at the full angle. However, the discriminant of this equation is positive, so this equation has three real roots (of which only one is the solution for the cosine of the one-third angle). None of these solutions is reducible to a real algebraic expression, as they use intermediate complex numbers under the cube roots.
Sine, cosine, and tangent of multiple angles
for nonnegative values of up through 
In each of these two equations, the first parenthesized term is a binomial coefficient, and the final trigonometric function equals one or minus one or zero so that half the entries in each of the sums are removed. The ratio of these formulae gives
cos(nx) can be computed from cos((n − 1)x), cos((n − 2)x), and cos(x) with
- cos(nx) = 2 · cos x · cos((n − 1)x) − cos((n − 2)x).
This can be proved by adding together the formulae
- cos((n − 1)x + x) = cos((n − 1)x) cos x − sin((n − 1)x) sin x
- cos((n − 1)x − x) = cos((n − 1)x) cos x + sin((n − 1)x) sin x.
It follows by induction that cos(nx) is a polynomial of the so-called Chebyshev polynomial of the first kind, see Chebyshev polynomials#Trigonometric definition.
Similarly, sin(nx) can be computed from sin((n − 1)x), sin((n − 2)x), and cos(x) with
- sin(nx) = 2 · cos x · sin((n − 1)x) − sin((n − 2)x).
This can be proved by adding formulae for sin((n − 1)x + x) and sin((n − 1)x − x).
Serving a purpose similar to that of the Chebyshev method, for the tangent we can write:
Tangent of an average
Setting either or to 0 gives the usual tangent half-angle formulae.
Viète's infinite product
Obtained by solving the second and third versions of the cosine double-angle formula.
Product-to-sum and sum-to-product identities
The product-to-sum identities or prosthaphaeresis formulae can be proven by expanding their right-hand sides using the angle addition theorems. See amplitude modulation for an application of the product-to-sum formulae, and beat (acoustics) and phase detector for applications of the sum-to-product formulae.