Even Multiple Angle Formula for Cosine

Theorem

 * $\displaystyle \cos 2 n \theta = \paren {-1}^n \prod_{k \mathop = 1}^n \paren {1 - \frac {\cos^2 \theta} {\map {\cos^2} {\frac {\paren {2 k - 1} \pi} {4 n} } } }$