## Precalculus (6th Edition)

$\{0^{o}, 90^{o}, 180^{o},270^{o}\}$
... zero factor principle ... 1. $\quad\sin^{2}\theta=0$ or 2.$\quad \cos^{2}\theta=0$ 1. $\quad\sin\theta=0\Rightarrow\theta=0^{o}, 180^{o}$ 2.$\quad \cos\theta=0\Rightarrow\theta=90^{o}, 270^{o}$ Solution set = $\{0^{o}, 90^{o}, 180^{o},270^{o}\}$