Answer
$\theta=\pi, \frac{\pi}{3},\frac{5\pi}{3}$
Work Step by Step
Recall $ cos2\theta=2cos^2\theta-1$; we rewrite the equation as: $2cos^2\theta+cos\theta-1=0$
Solving the equation leads to $ cos\theta=-1$ or $ cos\theta=\frac{1}{2}$
Within $[0,2\pi]$, $\theta=\pi, \frac{\pi}{3},\frac{5\pi}{3}$
