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

Theorem
For $x \in \left[{- \pi \,.\,.\, \pi}\right]$:
 * $\displaystyle \left\vert{x}\right\vert = \frac \pi 2 - \frac 4 \pi \sum_{n \mathop = 1}^\infty \frac {\cos\left({2 n - 1}\right) x} {\left({2 n - 1}\right)^2}$