\displaystyle P = 2 \left( \frac{1}{2} \int \limits_0^{\pi/3} 9 \cos^2 \varphi \, d\varphi - \frac{1}{2} \int \limits_0^{\pi/3} (1 + \cos \varphi)^2 \, d\varphi \right) = \ldots = \pi