Answer
$\frac{\pi}{3},\frac{2\pi}{3},\frac{4\pi}{3},\frac{5\pi}{3}$
Work Step by Step
1. $sec^2(\theta)=4\Longrightarrow cos(\theta)=\pm\frac{1}{2}$,
2. For $cos(\theta)=\frac{1}{2}$, we have $\theta=2k\pi+\frac{\pi}{3}$ or $\theta=2k\pi+\frac{5\pi}{3}$
3. For $cos(\theta)=-\frac{1}{2}$, we have $\theta=2k\pi+\frac{2\pi}{3}$ or $\theta=2k\pi+\frac{4\pi}{3}$
4. Within $[0,2\pi)$, we have $\theta=\frac{\pi}{3},\frac{2\pi}{3},\frac{4\pi}{3},\frac{5\pi}{3}$
