\displaystyle P = \frac{1}{2} \int \limits_{-\pi / 3}^{\pi / 3} (3 \cos \varphi)^2 \, d \varphi - \frac{1}{2} \int \limits_{-\pi / 3}^{\pi / 3} (1 + \cos \varphi)^2 \, d \varphi