Answer
$\frac{\pi}{2},\frac{2\pi}{3},\frac{4\pi}{3},\frac{3\pi}{2}$
Work Step by Step
1. $2cos^2\theta+cos\theta=0 \Longrightarrow cos\theta=0, -\frac{1}{2}$,
2. $cos\theta=0\Longrightarrow \theta=\frac{\pi}{2},\frac{3\pi}{2}$ in $[0,2\pi)$,
3. $cos\theta=-\frac{1}{2}\Longrightarrow \theta=\frac{2\pi}{3},\frac{4\pi}{3}$ in $[0,2\pi)$.
