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In mathematics, trigonometric identities are equalities that involve trigonometric functions that are true for all values of the occurring variables. 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 trick involves first using the substitution rule with a trigonometric function, and then simplifying the resulting integral with a trigonometric identity.

Notation

To avoid the confusion caused by the ambiguity of sin(x), the reciprocals and inverses of trigonometric functions are often displayed as in this table. In representing the cosecant function, the longer form 'cosec' is sometimes used in place of 'csc'.

Different angular measures can be appropriate in different situations. This table shows some of the more common systems. Radians is the default angular measure and is the one you use if you use the exponential definitions. All angular measures are unitless.

Degrees	30	45	60	90	120	180	270	360
Radians	${\displaystyle \pi /6}$	${\displaystyle \pi /4}$	${\displaystyle \pi /3}$	${\displaystyle \pi /2}$	${\displaystyle 2\pi /3}$	${\displaystyle \pi }$	${\displaystyle 3\pi /2}$	${\displaystyle 2\pi }$
Grads	33 ⅓	50	66 ⅔	100	133 ⅓	200	300	400

Basic relationships

Pythagorean trigonometric identity	${\displaystyle \sin ^{2}\theta +\cos ^{2}\theta =1\,}$
Ratio identity	${\displaystyle \tan \theta ={\frac {\sin \theta }{\cos \theta }}}$

From the two identities above, the following table can be extrapolated.

Each trigonometric function in terms of the other five.

Function	sin	cos	tan	csc	sec	cot
${\displaystyle \sin \theta =}$	${\displaystyle \sin \theta \ }$	${\displaystyle {\sqrt {1-\cos ^{2}\theta }}}$	${\displaystyle {\frac {\tan \theta }{\sqrt {1+\tan ^{2}\theta }}}}$	${\displaystyle {\frac {1}{\csc \theta }}}$	${\displaystyle {\frac {\sqrt {\sec ^{2}\theta -1}}{\sec \theta }}}$	${\displaystyle {\frac {1}{\sqrt {1+\cot ^{2}\theta }}}}$
${\displaystyle \cos \theta =}$	${\displaystyle {\sqrt {1-\sin ^{2}\theta }}}$	${\displaystyle \cos \theta \ }$	${\displaystyle {\frac {1}{\sqrt {1+\tan ^{2}\theta }}}}$	${\displaystyle {\frac {\sqrt {\csc ^{2}\theta -1}}{\csc \theta }}}$	${\displaystyle {\frac {1}{\sec \theta }}}$	${\displaystyle {\frac {\cot \theta }{\sqrt {1+\cot ^{2}\theta }}}}$
${\displaystyle \tan \theta =}$	${\displaystyle {\frac {\sin \theta }{\sqrt {1-\sin ^{2}\theta }}}}$	${\displaystyle {\frac {\sqrt {1-\cos ^{2}\theta }}{\cos \theta }}}$	${\displaystyle \tan \theta \ }$	${\displaystyle {\frac {1}{\sqrt {\csc ^{2}\theta -1}}}}$	${\displaystyle {\sqrt {\sec ^{2}\theta -1}}}$	${\displaystyle {\frac {1}{\cot \theta }}}$
${\displaystyle \csc \theta =}$	${\displaystyle {1 \over \sin \theta }}$	${\displaystyle {1 \over {\sqrt {1-\cos ^{2}\theta }}}}$	${\displaystyle {{\sqrt {1+\tan ^{2}\theta }} \over \tan \theta }}$	${\displaystyle \csc \theta \ }$	${\displaystyle {\sec \theta \over {\sqrt {\sec ^{2}\theta -1}}}}$	${\displaystyle {\sqrt {1+\cot ^{2}\theta }}}$
${\displaystyle \sec \theta =}$	${\displaystyle {1 \over {\sqrt {1-\sin ^{2}\theta }}}}$	${\displaystyle {1 \over \cos \theta }}$	${\displaystyle {\sqrt {1+\tan ^{2}\theta }}}$	${\displaystyle {\csc \theta \over {\sqrt {\csc ^{2}\theta -1}}}}$	${\displaystyle \sec \theta \ }$	${\displaystyle {{\sqrt {1+\cot ^{2}\theta }} \over \cot \theta }}$
${\displaystyle \cot \theta =}$	${\displaystyle {{\sqrt {1-\sin ^{2}\theta }} \over \sin \theta }}$	${\displaystyle {\cos \theta \over {\sqrt {1-\cos ^{2}\theta }}}}$	${\displaystyle {1 \over \tan \theta }}$	${\displaystyle {\sqrt {\csc ^{2}\theta -1}}}$	${\displaystyle {1 \over {\sqrt {\sec ^{2}\theta -1}}}}$	${\displaystyle \cot \theta \ }$

Historic shorthands

Rarely used today, the versine, coversine, haversine, and exsecant have been defined as below and used in navigation, for example the haversine formula was used to calculate the distance between two points on a sphere.

Name	Value
${\displaystyle {\textrm {versin}}\,\theta }$	${\displaystyle 1-\cos \theta \,}$
${\displaystyle {\textrm {coversin}}\,\theta }$	${\displaystyle 1-\sin \theta \,}$
${\displaystyle {\textrm {haversin}}\,\theta }$	${\displaystyle {\tfrac {1}{2}}{\textrm {versin}}\theta \,}$
${\displaystyle {\textrm {exsec}}\,\theta \,}$	${\displaystyle \sec \theta -1\,}$

Symmetry, shifts, and periodicity

By examining the unit circle, the following properties of the trigonometric functions can be established.

Symmetry

When the trigonometric functions are reflected from certain values of ${\displaystyle \theta }$, The result is often one of the other trigonometric functions. This leads to the following identities:

Reflected in ${\displaystyle \theta =0}$	Reflected in ${\displaystyle \theta =\pi /2}$	Reflected in ${\displaystyle \theta =\pi }$
${\displaystyle {\begin{aligned}\sin(0-\theta )&=-\sin \theta \\\cos(0-\theta )&=+\cos \theta \\\tan(0-\theta )&=-\tan \theta \\\csc(0-\theta )&=-\csc \theta \\\sec(0-\theta )&=+\sec \theta \\\cot(0-\theta )&=-\cot \theta \end{aligned}}}$	${\displaystyle {\begin{aligned}\sin({\tfrac {\pi }{2}}-\theta )&=+\cos \theta \\\cos({\tfrac {\pi }{2}}-\theta )&=+\sin \theta \\\tan({\tfrac {\pi }{2}}-\theta )&=+\cot \theta \\\csc({\tfrac {\pi }{2}}-\theta )&=+\sec \theta \\\sec({\tfrac {\pi }{2}}-\theta )&=+\csc \theta \\\cot({\tfrac {\pi }{2}}-\theta )&=+\tan \theta \end{aligned}}}$	${\displaystyle {\begin{aligned}\sin(\pi -\theta )&=+\sin \theta \\\cos(\pi -\theta )&=-\cos \theta \\\tan(\pi -\theta )&=-\tan \theta \\\csc(\pi -\theta )&=+\csc \theta \\\sec(\pi -\theta )&=-\sec \theta \\\cot(\pi -\theta )&=-\cot \theta \\\end{aligned}}}$

Shifts and periodicity

By shifting the function round by certain angles, it is often possible to find different trigonometric functions that express the result more simply. Some examples of this are given shown by shifting functions round by π/2, π and 2π radians. Because the periods of these functions are either π or 2π, there are cases where the new function is exactly the same as the old function without the shift.

Shift by π/2	Shift by π Period for tan and cot	Shift by 2π Period for sin, cos, csc and sec
${\displaystyle {\begin{aligned}\sin(\theta +{\tfrac {\pi }{2}})&=+\cos \theta \\\cos(\theta +{\tfrac {\pi }{2}})&=-\sin \theta \\\tan(\theta +{\tfrac {\pi }{2}})&=-\cot \theta \\\csc(\theta +{\tfrac {\pi }{2}})&=+\sec \theta \\\sec(\theta +{\tfrac {\pi }{2}})&=-\csc \theta \\\cot(\theta +{\tfrac {\pi }{2}})&=-\tan \theta \end{aligned}}}$	${\displaystyle {\begin{aligned}\sin(\theta +\pi )&=-\sin \theta \\\cos(\theta +\pi )&=-\cos \theta \\\tan(\theta +\pi )&=+\tan \theta \\\csc(\theta +\pi )&=-\csc \theta \\\sec(\theta +\pi )&=-\sec \theta \\\cot(\theta +\pi )&=+\cot \theta \\\end{aligned}}}$	${\displaystyle {\begin{aligned}\sin(\theta +2\pi )&=+\sin \theta \\\cos(\theta +2\pi )&=+\cos \theta \\\tan(\theta +2\pi )&=+\tan \theta \\\csc(\theta +2\pi )&=+\csc \theta \\\sec(\theta +2\pi )&=+\sec \theta \\\cot(\theta +2\pi )&=+\cot \theta \end{aligned}}}$

Angle sum and difference identities

These are also known as the addition and subtraction theorems or formulæ. The quickest way to prove these is Euler's formula.

Sine	${\displaystyle \sin(\theta \pm \phi )=\sin \theta \cos \phi \pm \cos \theta \sin \phi \,}$	Note: From Plus-minus sign#Minus-plus sign. ${\displaystyle {\begin{aligned}x\pm y=a\pm b&\Rightarrow \ x+y=a+b\\&{\mbox{and}}\ x-y=a-b\end{aligned}}}$ ${\displaystyle {\begin{aligned}x\pm y=a\mp b&\Rightarrow \ x+y=a-b\\&{\mbox{and}}\ x-y=a+b\end{aligned}}}$
Cosine	${\displaystyle \cos(\theta \pm \phi )=\cos \theta \cos \phi \mp \sin \theta \sin \phi \,}$
Tangent	${\displaystyle \tan(\theta \pm \phi )={\frac {\tan \theta \pm \tan \phi }{1\mp \tan \theta \tan \phi }}}$

Sines and cosines of sums of infinitely many terms

${\displaystyle \sin \left(\sum _{i=1}^{\infty }\theta _{i}\right)=\sum _{\mathrm {odd} \ k\geq 1}(-1)^{(k-1)/2}\sum _{|A|=k}\left(\prod _{i\in A}\sin \theta _{i}\prod _{i\not \in A}\cos \theta _{i}\right)}$

${\displaystyle \cos \left(\sum _{i=1}^{\infty }\theta _{i}\right)=\sum _{\mathrm {even} \ k\geq 0}~(-1)^{k/2}~~\sum _{|A|=k}\left(\prod _{i\in A}\sin \theta _{i}\prod _{i\not \in A}\cos \theta _{i}\right)}$

where "|A| = k" means the index A runs through the set of all subsets of size k of the set { 1, 2, 3, ... }.

In these two identities an asymmetry appears that is not seen in the case of sums of finitely many terms: in each product, there are only finitely many sine factors and cofinitely many cosine factors.

If only finitely many of the terms θ_i are nonzero, then only finitely many of the terms on the right side will be nonzero because sine factors will vanish, and in each term, all but finitely many of the cosine factors will be unity.

Tangents of sums of finitely many terms

Let x_i = tan(θ_i ), for i = 1, ..., n. Let e_k be the kth-degree elementary symmetric polynomial in the variables x_i, i = 1, ..., n, k = 0, ..., n. Then

${\displaystyle \tan(\theta _{1}+\cdots +\theta _{n})={\frac {e_{1}-e_{3}+e_{5}-\cdots }{e_{0}-e_{2}+e_{4}-\cdots }},}$

the number of terms depending on n.

For example,

${\displaystyle {\begin{aligned}\tan(\theta _{1}+\theta _{2}+\theta _{3})&{}={\frac {e_{1}-e_{3}}{e_{0}-e_{2}}}={\frac {(x_{1}+x_{2}+x_{3})\ -\ (x_{1}x_{2}x_{3})}{1\ -\ (x_{1}x_{2}+x_{1}x_{3}+x_{2}x_{3})}},\\\\\tan(\theta _{1}+\theta _{2}+\theta _{3}+\theta _{4})&{}={\frac {e_{1}-e_{3}}{e_{0}-e_{2}+e_{4}}}\\\\&{}={\frac {(x_{1}+x_{2}+x_{3}+x_{4})\ -\ (x_{1}x_{2}x_{3}+x_{1}x_{2}x_{4}+x_{1}x_{3}x_{4}+x_{2}x_{3}x_{4})}{1\ -\ (x_{1}x_{2}+x_{1}x_{3}+x_{1}x_{4}+x_{2}x_{3}+x_{2}x_{4}+x_{3}x_{4})\ +\ (x_{1}x_{2}x_{3}x_{4})}},\end{aligned}}}$

and so on. The general case can be proved by mathematical induction.

Multiple-angle formulae

Tn is the nth Chebyshev polynomial	${\displaystyle \cos n\theta =T_{n}(\cos \theta )\,}$
Sn is the nth spread polynomial	${\displaystyle \sin ^{2}n\theta =S_{n}(\sin ^{2}\theta )\,}$
de Moivre's formula, ${\displaystyle i}$ is the Imaginary unit	${\displaystyle \cos n\theta +i\sin n\theta =(\cos(\theta )+i\sin(\theta ))^{n}\,}$

See also Dirichlet kernel.

Double, triple and half-angle formulae

These can be shown by using either the sum and difference identities or the multiple-angle formulae.

Double-angle formulae
${\displaystyle {\begin{aligned}\sin 2\theta &=2\sin \theta \cos \theta \ \\&={\frac {2\tan \theta }{1+\tan ^{2}\theta }}\end{aligned}}}$	${\displaystyle {\begin{aligned}\cos 2\theta &=\cos ^{2}\theta -\sin ^{2}\theta \\&=2\cos ^{2}\theta -1\\&=1-2\sin ^{2}\theta \\&={\frac {1-\tan ^{2}\theta }{1+\tan ^{2}\theta }}\end{aligned}}}$	${\displaystyle \tan 2\theta ={\frac {2\tan \theta }{1-\tan ^{2}\theta }}\,}$	${\displaystyle \cot 2\theta ={\frac {\cot \theta -\tan \theta }{2}}\,}$
Triple-angle formulae
${\displaystyle \sin 3\theta =3\sin \theta -4\sin ^{3}\theta \,}$	${\displaystyle \cos 3\theta =4\cos ^{3}\theta -3\cos \theta \,}$	${\displaystyle \tan 3\theta ={\frac {3\tan \theta -\tan ^{3}\theta }{1-3\tan ^{2}\theta }}}$
Half-angle formulae
${\displaystyle \sin {\tfrac {\theta }{2}}=\pm \,{\sqrt {\frac {1-\cos \theta }{2}}}}$	${\displaystyle \cos {\tfrac {\theta }{2}}=\pm \,{\sqrt {\frac {1+\cos \theta }{2}}}}$	${\displaystyle {\begin{aligned}\tan {\tfrac {\theta }{2}}&=\csc \theta -\cot \theta \\&=\pm \,{\sqrt {1-\cos \theta \over 1+\cos \theta }}\\&={\frac {\sin \theta }{1+\cos \theta }}\\&={\frac {1-\cos \theta }{\sin \theta }}\end{aligned}}}$	${\displaystyle \cot {\tfrac {\theta }{2}}=\csc \theta +\cot \theta }$

See also Tangent half-angle formula.

Power-reduction formulae

Obtained by solving the second and third versions of the cosine double-angle formula.

Sine	${\displaystyle \sin ^{2}\theta ={\frac {1-\cos 2\theta }{2}}}$	${\displaystyle \sin ^{3}\theta ={\frac {3\sin \theta -\sin 3\theta }{4}}}$
Cosine	${\displaystyle \cos ^{2}\theta ={\frac {1+\cos 2\theta }{2}}}$	${\displaystyle \cos ^{3}\theta ={\frac {3\cos \theta +\cos 3\theta }{4}}}$
Other	${\displaystyle \sin ^{2}\theta \cos ^{2}\theta ={\frac {1-\cos 4\theta }{8}}}$	${\displaystyle \sin ^{3}\theta \cos ^{3}\theta ={\frac {\sin ^{3}2\theta }{8}}}$

Product-to-sum and sum-to-product identities

The product-to-sum identies can be proven by expanding their right-hand sides using the angle addition theorems.

Product-to-sum ${\displaystyle \cos \theta \cos \phi ={\cos(\theta -\phi )+\cos(\theta +\phi ) \over 2}}$ ${\displaystyle \sin \theta \sin \phi ={\cos(\theta -\phi )-\cos(\theta +\phi ) \over 2}}$ ${\displaystyle \sin \theta \cos \phi ={\sin(\theta +\phi )+\sin(\theta -\phi ) \over 2}}$

Sum-to-product ${\displaystyle \sin \theta +\sin \phi =2\sin \left({\frac {\theta +\phi }{2}}\right)\cos \left({\frac {\theta -\phi }{2}}\right)}$ ${\displaystyle \cos \theta +\cos \phi =2\cos \left({\frac {\theta +\phi }{2}}\right)\cos \left({\frac {\theta -\phi }{2}}\right)}$ ${\displaystyle \cos \theta -\cos \phi =-2\sin \left({\theta +\phi \over 2}\right)\sin \left({\theta -\phi \over 2}\right)}$ ${\displaystyle \sin \theta -\sin \phi =2\cos \left({\theta +\phi \over 2}\right)\sin \left({\theta -\phi \over 2}\right)\;}$

Other related identities

If x, y, and z are the three angles of any triangle, or in other words

${\displaystyle {\mbox{if }}x+y+z=\pi ={\mbox{half circle,}}\,}$

${\displaystyle {\mbox{then }}\tan(x)+\tan(y)+\tan(z)=\tan(x)\tan(y)\tan(z).\,}$

(If any of x, y, z is a right angle, one should take both sides to be ∞. This is neither +∞ nor −∞; for present purposes it makes sense to add just one point at infinity to the real line, that is approached by tan(θ) as tan(θ) either increases through positive values or decreases through negative values. This is a one-point compactification of the real line.)

${\displaystyle {\mbox{If }}x+y+z=\pi ={\mbox{half circle,}}\,}$

${\displaystyle {\mbox{then }}\sin(2x)+\sin(2y)+\sin(2z)=4\sin(x)\sin(y)\sin(z).\,}$

Ptolemy's theorem

${\displaystyle {\mbox{If }}w+x+y+z=\pi ={\mbox{half circle,}}\,}$

${\displaystyle {\begin{aligned}{\mbox{then }}&\sin(w+x)\sin(x+y)\\&{}=\sin(x+y)\sin(y+z)\\&{}=\sin(y+z)\sin(z+w)\\&{}=\sin(z+w)\sin(w+x)=\sin(w)\sin(y)+\sin(x)\sin(z).\end{aligned}}}$

(The first three equalities are trivial; the fourth is the substance of this identity.) Essentially this is Ptolemy's theorem adapted to the language of trigonometry.

Linear combinations

For some purposes it is important to know that any linear combination of sine waves of the same period but different phase shifts is also a sine wave with the same period, but a different phase shift. In the case of a linear combination of a sine and cosine wave, we have

${\displaystyle a\sin x+b\cos x={\sqrt {a^{2}+b^{2}}}\cdot \sin(x+\varphi )\,}$

where

${\displaystyle \varphi =\arctan \left({\frac {b}{a}}\right)}$

More generally, for an arbitrary phase shift, we have

${\displaystyle a\sin x+b\sin(x+\alpha )=c\sin(x+\beta )\,}$

where

${\displaystyle c={\sqrt {a^{2}+b^{2}+2ab\cos \alpha }},}$

and

${\displaystyle \beta ={\rm {arctan}}\left({\frac {b\sin \alpha }{a+b\cos \alpha }}\right).}$

• note: arcsin, arccos, arctan are all inverses.

Other sums of trigonometric functions

Sum of sines and cosines with arguments in arithmetic progression:

${\displaystyle \sin {\varphi }+\sin {(\varphi +\alpha )}+\sin {(\varphi +2\alpha )}+\cdots +\sin {(\varphi +n\alpha )}={\frac {\sin {\left({\frac {(n+1)\alpha }{2}}\right)}\cdot \sin {(\varphi +{\frac {n\alpha }{2}})}}{\sin {\frac {\alpha }{2}}}}.}$

${\displaystyle \cos {\varphi }+\cos {(\varphi +\alpha )}+\cos {(\varphi +2\alpha )}+\cdots +\cos {(\varphi +n\alpha )}={\frac {\sin {\left({\frac {(n+1)\alpha }{2}}\right)}\cdot \cos {(\varphi +{\frac {n\alpha }{2}})}}{\sin {\frac {\alpha }{2}}}}.}$

For any a and b:

${\displaystyle a\cos(x)+b\sin(x)={\sqrt {a^{2}+b^{2}}}\cos(x-\arctan(b,a))\;}$

where arctan(y, x) is the generalization of arctan(y/x) which covers the entire circular range (see also the account of this same identity in "symmetry, periodicity, and shifts" above for this generalization of arctan).

${\displaystyle \tan(x)+\sec(x)=\tan \left({x \over 2}+{\pi \over 4}\right).}$

The above identity is sometimes convenient to know when thinking about the Gudermanian function.

If ${\displaystyle x}$, ${\displaystyle y}$, and ${\displaystyle z}$ are the three angles of any triangle, i.e. if ${\displaystyle x+y+z=\pi }$, then

${\displaystyle \cot(x)\cot(y)+\cot(y)\cot(z)+\cot(z)\cot(x)=1.\,}$

Inverse trigonometric functions

${\displaystyle \arcsin(x)+\arccos(x)=\pi /2\;}$

${\displaystyle \arctan(x)+\operatorname {arccot}(x)=\pi /2.\;}$

${\displaystyle \arctan(x)+\arctan(1/x)=\left\{{\begin{matrix}\pi /2,&{\mbox{if }}x>0\\-\pi /2,&{\mbox{if }}x<0\end{matrix}}\right.}$

${\displaystyle \arctan(x)+\arctan(y)=\arctan \left({\frac {x+y}{1-xy}}\right)+\left\{{\begin{matrix}\pi ,&{\mbox{if }}x,y>0\\-\pi ,&{\mbox{if }}x,y<0\\0,&{\mbox{otherwise }}\end{matrix}}\right.}$

Relation to the complex exponential function

${\displaystyle e^{ix}=\cos(x)+i\sin(x)\,}$ (Euler's formula),

${\displaystyle e^{-ix}=\cos(-x)+i\sin(-x)=\cos(x)-i\sin(x)\,}$

${\displaystyle e^{i\pi }=-1\,}$

${\displaystyle \cos(x)={\frac {e^{ix}+e^{-ix}}{2}}\;}$

${\displaystyle \sin(x)={\frac {e^{ix}-e^{-ix}}{2i}}\;}$

where i² = −1.

"cis"

It has been suggested that this section be split out into another article titled cis (mathematics). (Discuss)

Occasionally one sees the notation

${\displaystyle \operatorname {cis} (x)=\cos(x)+i\sin(x),\,}$

i.e. "cis" abbreviates "cos + i sin".

Convenience

This notation was more common in the post WWII era when type-writers were used to convey mathematical expressions. Superscripts are both offset vertically and smaller than 'cis' or 'exp'; hence, they can be problematic even for hand writing. For example e versus cis( x²) versus exp( ix²). For many readers, cis( x²) is the clearest, easiest to read of the three. And in fact it is composed of the minimum number of symbols: (3: cis,x,2) compared to (4: e,,i,x,2) and (4: exp,,i,x,2). It is therefore a more compact notation, regardless of the subjective reader.

It is also sometimes used to emphasize one method of viewing and dealing with a problem over an other. The mathematics of trigonometry and exponentials are related but not exactly the same. Exponential emphasizes the whole, where as cis and cos + i sin notations emphasis the parts. A sort of rhetorical technique for mathematicians, engineers, etc.

It also serves as a mnemonic.

Pedagogy

In some contexts, this notation may serve the pedagogical purpose of emphasizing that one has not yet proved that this is an exponential function. In doing trigonometry without complex numbers, one may prove the two identities

${\displaystyle \cos(x+y)=\cos(x)\cos(y)-\sin(x)\sin(y)=c_{1}c_{2}-s_{1}s_{2},\,}$

${\displaystyle \sin(x+y)=\sin(x)\cos(y)+\cos(x)\sin(y)=s_{1}c_{2}+c_{1}s_{2}.\,}$

Similarly in treating multiplication of complex numbers (with no involvement of trigonometry), one may observe that the real and imaginary parts of the product of c₁ + is₁ and c₂ + is₂ are respectively

${\displaystyle c_{1}c_{2}-s_{1}s_{2},\,}$

${\displaystyle s_{1}c_{2}+c_{1}s_{2}.\,}$

Thus one sees this same pattern arising in two disparate contexts:

• trigonometry without complex numbers, and
• complex numbers without trigonometry.

This coincidence can serve as a motivation for conjoining the two contexts and thereby discovering the trigonometric identity

${\displaystyle \operatorname {cis} (x+y)=\operatorname {cis} (x)\operatorname {cis} (y),\,}$

and observing that this identity for cis of a sum is simpler than the identities for sin and cos of a sum. Having proved this identity, one can challenge the students to recall which familiar sort of function satisfies this same functional equation

${\displaystyle f(x+y)=f(x)f(y).\,}$

The answer is exponential functions. That suggests that cis may be an exponential function

${\displaystyle \operatorname {cis} (x)=b^{x}.\,}$

Then the question is: what is the base b? The definition of cis and the local behavior of sin and cos near zero suggest that

${\displaystyle \operatorname {cis} (0+dx)=\operatorname {cis} (0)+i\,dx,}$

(where dx is an infinitesimal increment of x). Thus the rate of change at 0 is i, so the base should be e. Thus if this is an exponential function, then it must be

${\displaystyle \operatorname {cis} (x)=e^{ix}.\,}$

Infinite product formula

For applications to special functions, the following infinite product formulæ for trigonometric functions are useful:

The Gudermannian function

The Gudermannian function relates the circular and hyperbolic trigonometric functions without resorting to complex numbers; see that article for details.

Identities without variables

The curious identity

${\displaystyle \cos 20^{\circ }\cdot \cos 40^{\circ }\cdot \cos 80^{\circ }={\frac {1}{8}}}$

is a special case of an identity that contains one variable:

${\displaystyle \prod _{j=0}^{k-1}\cos(2^{j}x)={\frac {\sin(2^{k}x)}{2^{k}\sin(x)}}.}$

A similar-looking identity is

${\displaystyle \cos {\frac {\pi }{7}}\cos {\frac {2\pi }{7}}\cos {\frac {3\pi }{7}}={\frac {1}{8}},}$

and in addition

${\displaystyle \sin 20^{\circ }\cdot \sin 40^{\circ }\cdot \sin 80^{\circ }={\sqrt {3}}/8.}$

The following is perhaps not as readily generalized to an identity containing variables:

${\displaystyle \cos 24^{\circ }+\cos 48^{\circ }+\cos 96^{\circ }+\cos 168^{\circ }={\frac {1}{2}}.}$

Degree measure ceases to be more felicitous than radian measure when we consider this identity with 21 in the denominators:

${\displaystyle \cos \left({\frac {2\pi }{21}}\right)\,+\,\cos \left(2\cdot {\frac {2\pi }{21}}\right)\,+\,\cos \left(4\cdot {\frac {2\pi }{21}}\right)}$

${\displaystyle \,+\,\cos \left(5\cdot {\frac {2\pi }{21}}\right)\,+\,\cos \left(8\cdot {\frac {2\pi }{21}}\right)\,+\,\cos \left(10\cdot {\frac {2\pi }{21}}\right)={\frac {1}{2}}.}$

The factors 1, 2, 4, 5, 8, 10 may start to make the pattern clear: they are those integers less than 21/2 that are relatively prime to (or have no prime factors in common with) 21. The last several examples are corollaries of a basic fact about the irreducible cyclotomic polynomials: the cosines are the real parts of the zeroes of those polynomials; the sum of the zeroes is the Möbius function evaluated at (in the very last case above) 21; only half of the zeroes are present above. The two identities preceding this last one arise in the same fashion with 21 replaced by 10 and 15, respectively.

An efficient way to compute π is based on the following identity without variables, due to Machin:

${\displaystyle {\frac {\pi }{4}}=4\arctan {\frac {1}{5}}-\arctan {\frac {1}{239}}}$

or, alternatively, by using Euler's formula:

${\displaystyle {\frac {\pi }{4}}=5\arctan {\frac {1}{7}}+2\arctan {\frac {3}{79}}.}$

${\displaystyle {\begin{matrix}\sin 0&=&\sin 0^{\circ }&;=&0&=&\cos 90^{\circ }&=&\cos \left({\frac {\pi }{2}}\right)\\\\\sin \left({\frac {\pi }{6}}\right)&=&\sin 30^{\circ }&=&1/2&=&\cos 60^{\circ }&=&\cos \left({\frac {\pi }{3}}\right)\\\\\sin \left({\frac {\pi }{4}}\right)&=&\sin 45^{\circ }&=&{\sqrt {2}}/2&=&\cos 45^{\circ }&=&\cos \left({\frac {\pi }{4}}\right)\\\\\sin \left({\frac {\pi }{3}}\right)&=&\sin 60^{\circ }&=&{\sqrt {3}}/2&=&\cos 30^{\circ }&=&\cos \left({\frac {\pi }{6}}\right)\\\\\sin \left({\frac {\pi }{2}}\right)&=&\sin 90^{\circ }&=&1&=&\cos 0^{\circ }&=&\cos 0\end{matrix}}}$

${\displaystyle \sin {\frac {\pi }{7}}={\frac {\sqrt {7}}{6}}-{\frac {\sqrt {7}}{189}}\sum _{j=0}^{\infty }{\frac {(3j+1)!}{189^{j}j!\,(2j+2)!}}\!}$

${\displaystyle \sin {\frac {\pi }{18}}={\frac {1}{6}}\sum _{j=0}^{\infty }{\frac {(3j)!}{27^{j}j!\,(2j+1)!}}\!}$

With the golden ratio φ:

${\displaystyle \cos \left({\frac {\pi }{5}}\right)=\cos 36^{\circ }={{\sqrt {5}}+1 \over 4}=\varphi /2}$

${\displaystyle \sin \left({\frac {\pi }{10}}\right)=\sin 18^{\circ }={{\sqrt {5}}-1 \over 4}={\varphi -1 \over 2}={1 \over 2\varphi }}$

Also see exact trigonometric constants.

Calculus

In calculus the relations stated below require angles to be measured in radians; the relations would become more complicated if angles were measured in another unit such as degrees. If the trigonometric functions are defined in terms of geometry, their derivatives can be found by verifying two limits. The first is:

${\displaystyle \lim _{x\rightarrow 0}{\frac {\sin(x)}{x}}=1,}$

verified using the unit circle and squeeze theorem. It may be tempting to propose to use L'Hôpital's rule to establish this limit. However, if one uses this limit in order to prove that the derivative of the sine is the cosine, and then uses the fact that the derivative of the sine is the cosine in applying L'Hôpital's rule, one is reasoning circularly—a logical fallacy. The second limit is:

${\displaystyle \lim _{x\rightarrow 0}{\frac {1-\cos(x)}{x}}=0,}$

verified using the identity tan(x/2) = (1 − cos(x))/sin(x). Having established these two limits, one can use the limit definition of the derivative and the addition theorems to show that sin′(x) = cos(x) and cos′(x) = −sin(x). If the sine and cosine functions are defined by their Taylor series, then the derivatives can be found by differentiating the power series term-by-term.

${\displaystyle {d \over dx}\sin(x)=\cos(x)}$

The rest of the trigonometric functions can be differentiated using the above identities and the rules of differentiation:

${\displaystyle {\begin{matrix}{d \over dx}\sin x=&\cos x,&{d \over dx}\arcsin x=&{1 \over {\sqrt {1-x^{2}}}}\\\\{d \over dx}\cos x=&-\sin x,&{d \over dx}\arccos x=&{-1 \over {\sqrt {1-x^{2}}}}\\\\{d \over dx}\tan x=&\sec ^{2}x,&{d \over dx}\arctan x=&{1 \over 1+x^{2}}\\\\{d \over dx}\cot x=&-\csc ^{2}x,&{d \over dx}\operatorname {arccot} x=&{-1 \over 1+x^{2}}\\\\{d \over dx}\sec x=&\tan x\sec x,&{d \over dx}\operatorname {arcsec} x=&{1 \over |x|{\sqrt {x^{2}-1}}}\\\\{d \over dx}\csc x=&-\csc x\cot x,&{d \over dx}\operatorname {arccsc} x=&{-1 \over |x|{\sqrt {x^{2}-1}}}\end{matrix}}}$

The integral identities can be found in "list of integrals of trigonometric functions".

Implications

The fact that the differentiation of trigonometric functions (sine and cosine) results in linear combinations of the same two functions is of fundamental importance to many fields of mathematics, including differential equations and fourier transformations.

Exponential definitions

Function	Inverse Function
${\displaystyle \sin \theta ={\frac {e^{i\theta }-e^{-i\theta }}{2i}}\,}$	${\displaystyle \arcsin x=-i\ln \left(ix+{\sqrt {1-x^{2}}}\right)\,}$
${\displaystyle \cos \theta ={\frac {e^{i\theta }+e^{-i\theta }}{2}}\,}$	${\displaystyle \arccos x=-i\ln \left(x+{\sqrt {x^{2}-1}}\right)\,}$
${\displaystyle \tan \theta ={\frac {e^{i\theta }-e^{-i\theta }}{i(e^{i\theta }+e^{-i\theta })}}\,}$	${\displaystyle \arctan x={\frac {i\ln \left({\frac {i+x}{i-x}}\right)}{2}}\,}$
${\displaystyle \csc \theta ={\frac {2i}{e^{i\theta }-e^{-i\theta }}}\,}$	${\displaystyle \operatorname {arccsc} x=-i\ln \left({\tfrac {i}{x}}+{\sqrt {1-{\tfrac {1}{x^{2}}}}}\right)\,}$
${\displaystyle \sec \theta ={\frac {2}{e^{i\theta }+e^{-i\theta }}}\,}$	${\displaystyle \operatorname {arcsec} x=-i\ln \left({\tfrac {1}{x}}+{\sqrt {1-{\tfrac {i}{x^{2}}}}}\right)\,}$
${\displaystyle \cot \theta ={\frac {i(e^{i\theta }+e^{-i\theta })}{e^{i\theta }-e^{-i\theta }}}\,}$	${\displaystyle \operatorname {arccot} x={\frac {i\ln \left({\frac {i-x}{i+x}}\right)}{2}}\,}$
${\displaystyle \operatorname {cis} \,\theta =e^{i\theta }\,}$	${\displaystyle \operatorname {arccis} \,x={\frac {\ln x}{i}}\,}$

Miscellaneous

Dirichlet kernel

The Dirichlet kernel D_n(x) is the function occurring on both sides of the next identity:

${\displaystyle 1+2\cos(x)+2\cos(2x)+2\cos(3x)+\cdots +2\cos(nx)={\frac {\sin \left(\left(n+{\frac {1}{2}}\right)x\right)}{\sin(x/2)}}.}$

The convolution of any integrable function of period 2π with the Dirichlet kernel coincides with the function's nth-degree Fourier approximation. The same holds for any measure or generalized function.

Extension of half-angle formulae

If we set

${\displaystyle t=\tan \left({\frac {x}{2}}\right),}$

then

${\displaystyle \sin(x)={\frac {2t}{1+t^{2}}}}$

and

${\displaystyle \cos(x)={\frac {1-t^{2}}{1+t^{2}}}}$

and

${\displaystyle e^{ix}={\frac {1+it}{1-it}}.}$

where eix is the same thing as cis(x).

This substitution of t for tan(x/2), with the consequent replacement of sin(x) by 2t/(1 + t²) and cos(x) by (1 − t²)/(1 + t²) is useful in calculus for converting rational functions in sin(x) and cos(x) to functions of t in order to find their antiderivatives. For more information see tangent half-angle formula.

See also

• Proofs of trigonometric identities
• Uses of trigonometry
• Tangent half-angle formula
• Law of cosines
• Law of sines
• Law of tangents
• Pythagorean theorem
• Exact trigonometric constants
• Derivatives of trigonometric functions

References

1. ^ Weisstein, Eric W. "Trigonometry". MathWorld.
2. ^ Weisstein, Eric W. "Trigonometric Addition Formulas". MathWorld.
3. ^ Weisstein, Eric W. "Multiple-Angle Formulas". MathWorld.
4. Weisstein, Eric W. "Double-Angle Formulas". MathWorld.
5. Weisstein, Eric W. "Half-Angle Formulas". MathWorld.

External links

• Mathematical identities
• Lists
• Trigonometry