Cosine of Right Angle

Theorem

 * $\cos 90^\circ = \cos \dfrac \pi 2 = 0$

where $\cos$ denotes cosine.

Proof
A direct implementation of Cosine of Multiple of Pi Plus Half:
 * $\forall n \in \Z: \cos \left({n + \dfrac 1 2}\right) \pi = 0$

In this case, $n = 0$ and so:
 * $\cos \dfrac 1 2 \pi = 0$