Wallis Cosine Formula

$\begin{displaymath} \int^{\pi /2}_0 \cos^n x\,dx = \cases{ {\pi\over 2} {1\cdot... ...-1)\over 1\cdot 3\cdot 5\cdots n} & for $n = 3$, 5, \dots.\cr} \end{displaymath}$

See also Wallis Formula, Wallis Sine Formula