Definite Integral from 0 to Half Pi of Odd Power of Cosine x

Theorem
Let $n \in \Z_{\ge 0}$ be a positive integer.

Then:
 * $\ds \int_0^{\frac \pi 2} \cos^{2 n + 1} x \rd x = \dfrac {\paren {2^n n!}^2} {\paren {2 n + 1}!}$