Answer
$\frac{\pi}{3},\pi,\frac{5\pi}{3}$
Work Step by Step
1. $2cos^2\theta+cos\theta-1=0 \Longrightarrow (cos\theta+1)(2cos\theta-1)=0 \Longrightarrow cos\theta=-1,\frac{1}{2}$,
2. $cos\theta=-1 \Longrightarrow \theta=\pi$ in $[0,2\pi)$,
3. $cos\theta=\frac{1}{2} \Longrightarrow \theta=\frac{\pi}{3},\frac{5\pi}{3}$ in $[0,2\pi)$.
