$ f(t) = \sin^2(t) $
$ E = \int_{t_1}^{t_2}\!|f(t)|^2\ dt $
$ E = \int_{t_1}^{t_2}\!|\sin^2(t)|^2\ dt $
$ E = \int_{0}^{2\pi}\!|\sin^4(t)|\ dt $
More to come ....
