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The trigonometric functions are periodic with common period , so for values of θ outside the interval (,], they take repeating values (see § Shifts and periodicity above). Angle sum and difference identities
The fixed point iteration x n+1 = cos(x n) with initial value x 0 = −1 converges to the Dottie number. Zero is the only real fixed point of the sine function; in other words the only intersection of the sine function and the identity function is sin ( 0 ) = 0 {\displaystyle \sin(0)=0} .
The original proof is based on the Taylor series expansions of the exponential function e z (where z is a complex number) and of sin x and cos x for real numbers x . In fact, the same proof shows that Euler's formula is even valid for all complex numbers x.
All trigonometric functions listed have period , unless otherwise stated. For the following trigonometric functions: U n is the n th up/down number, B n is the n th Bernoulli number in Jacobi elliptic functions, = ()
If () is a function with period , then (), where is a non-zero real number such that is within the domain of , is periodic with period . For example, f ( x ) = sin ( x ) {\displaystyle f(x)=\sin(x)} has period 2 π {\displaystyle 2\pi } and, therefore, sin ( 5 x ) {\displaystyle \sin(5x)} will have period 2 π 5 {\textstyle {\frac {2\pi ...
Identity 1: + = The following two results follow from this and the ratio identities. To obtain the first, divide both sides of + = by ; for the second, divide by .
The red section on the right, d, is the difference between the lengths of the hypotenuse, H, and the adjacent side, A.As is shown, H and A are almost the same length, meaning cos θ is close to 1 and θ 2 / 2 helps trim the red away.
A trigonometric polynomial can be considered a periodic function on the real line, with period some divisor of , or as a function on the unit circle.. Trigonometric polynomials are dense in the space of continuous functions on the unit circle, with the uniform norm; [4] this is a special case of the Stone–Weierstrass theorem.