Fourier Series/Absolute Value of x over Minus Pi to Pi

Theorem
For $x \in \openint {-\pi} \pi$:


 * $\dis \size x = \frac \pi 2 - \frac 4 \pi \sum_{n \mathop = 1}^\infty \frac {\map \cos {2 n - 1} x} {\paren {2 n - 1}^2}$