## Algebra and Trigonometry 10th Edition

$\theta=120°$ $\theta=240°$
Let $\theta'$ be a refernce angle such that $\theta'$ lies in the first quadrant and: $cos~\theta'=\frac{\sqrt 3}{2}$ Then: $\theta'=60°$ If $cos~\theta\lt0$, then $\theta$ lies in the second or in the third quadrant: $\theta=180°-\theta'=180°-60°=120°$ or $\theta=180°+\theta'=180°+60°=240 °$