## Calculus (3rd Edition)

$\frac{5\pi}{4}$
$\frac{13\pi}{4}=3\pi+ \frac{\pi}{4}=(2\pi+\pi+\frac{\pi}{4})=\pi+\frac{\pi}{4}$ (as angle repeats after $2\pi$). So, angle $\frac{13\pi}{4}= \pi+\frac{\pi}{4}=\frac{5\pi}{4}$ and $\frac{5\pi}{4}$ is in between 0 and $2\pi$.