Cosine Function is Even

Theorem
For all $z \in \C$:
 * $\cos \paren {-z} = \cos z$

That is, the cosine function is even.

Also see

 * Sine Function is Odd
 * Tangent Function is Odd
 * Cotangent Function is Odd
 * Secant Function is Even
 * Cosecant Function is Odd


 * Hyperbolic Cosine Function is Even