$<P> = \lim_{\tau \to \infty} \frac{1}{\tau} \int_{0}^{\tau} P(t) dt = 0$

$<P> = \lim_{\tau \to \infty} \frac{1}{\tau} \int_{0}^{\tau} P(t) dt = 0$

you also can take mean value in one period by take $\tau \to T$ where $T = \frac{2\pi}{2\omega}$ is the period of the oscillator.