Cosine of Straight Angle

Theorem

 * $\cos 180^\circ = \cos \pi = - 1$

where $\cos$ denotes cosine.

Proof
A direct implementation of Cosine of Multiple of Pi:
 * $\forall n \in \Z: \cos n \pi = \left({-1}\right)^n$

In this case, $n = 1$ and so:
 * $\cos \pi = -1^1 = -1$