## Precalculus (6th Edition) Blitzer

$210^{\circ}=\dfrac{7\pi}{6}$
$\dfrac{31 \pi}{6}=930 °$ Subtract $720 ^{\circ}$ (twice $360^{\circ}$) from $\dfrac{31 \pi}{6}=930 ^{\circ}$ to find the positive co-terminal angle less than $360^{\circ}$. $930^{\circ}- 720^{\circ} =210^{\circ}=\dfrac{7\pi}{6}$