Answer
$\frac{\pi}{3}, \frac{2\pi}{3},\frac{4\pi}{3},\frac{5\pi}{3} $
Work Step by Step
Using trigonometric identities, we have
$cos(2\theta)=-\frac{1}{2} \longrightarrow 2\theta=\pi-\frac{\pi}{3}, \pi+\frac{\pi}{3}, 2\pi+\pi-\frac{\pi}{3}, 2\pi+\pi+\frac{\pi}{3}, \longrightarrow \theta=\frac{\pi}{3}, \frac{2\pi}{3},\frac{4\pi}{3},\frac{5\pi}{3} $
