Fourier Series/Fourth Power of x over Minus Pi to Pi

Theorem
For $x \in \left[{-\pi \,.\,.\, \pi}\right]$:
 * $\displaystyle x^4 = \frac {\pi^4} 5 + \sum_{n \mathop = 1}^\infty \frac {8 n^2 \pi^2 - 48} {n^4} \cos \left({n \pi}\right) \cos \left({n x}\right)$