Answer
$\frac{\pi}{3}, \frac{2\pi}{3},\frac{4\pi}{3}, \frac{5\pi}{3}$
Work Step by Step
$cos(2\theta)=-\frac{1}{2}$,
$2\theta=2k\pi+\frac{2\pi}{3}$ or $2\theta=2k\pi+\frac{4\pi}{3}$ (k is an integer),
$\theta=\frac{\pi}{3}, \frac{2\pi}{3},\frac{4\pi}{3}, \frac{5\pi}{3}$ in $[0,2\pi)$.
