## Calculus (3rd Edition)

$\frac{\pi}{3}$, $\frac{5\pi}{3}$
$\sec t=\frac{1}{\cos t}=2$ $\implies \cos t=\frac{1}{2}$ From Figure 22, we find that $\cos \frac{\pi}{3}=\cos \frac{5\pi}{3}=\frac{1}{2}$ Therefore, the required angles are $\frac{\pi}{3}$ and $\frac{5\pi}{3}$.