Cosine of Right Angle

Theorem

 * $\cos 90 \degrees = \cos \dfrac \pi 2 = 0$

where $\cos$ denotes cosine.

Proof
A direct implementation of Cosine of Half-Integer Multiple of Pi:
 * $\forall n \in \Z: \map \cos {n + \dfrac 1 2} \pi = 0$

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

Also see

 * Sine of Right Angle
 * Tangent of Right Angle
 * Cotangent of Right Angle
 * Secant of Right Angle
 * Cosecant of Right Angle