$\int_{-\pi/3}^{\pi/3} 4\cos^2(3\theta) d\theta$

$\frac{1}{2} \int_{-\pi/6}^{\pi/6} 4\cos^2(3\theta) d\theta$

$\int_{0}^{\pi/3} 2\cos^2(3\theta) d\theta$

$\frac{1}{2} \int_{-\pi/2}^{\pi/2} 4\cos^2(3\theta) d\theta$