$ x(t) = cos(2 \pi t) $ from $ 0 $ to $ 5 \pi $
$ E = \int_{0}^{5 \pi}{|cos(2 \pi t)|^2dt} $
$ = \int_{0}^{5 \pi}{[1 + cos(4 \pi t)]dt} $
$ =\frac{5 \pi}{2} + \frac{1}{8 \pi} sin(20 \pi^2) $
