Even Multiple Angle Formula for Cosine

Theorem

 * $\displaystyle \cos 2 n \theta = \left({-1}\right)^n \prod_{k \mathop = 1}^n \left({1 - \frac {\cos^2 \theta}{\cos^2 \left({\frac {\left({2 k - 1}\right) \pi} {4 n} }\right)} }\right)$