#### Answer

$\{0^{o}, 90^{o}, 180^{o},270^{o}\}$

#### Work Step by Step

... 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 = $\{0^{o}, 90^{o}, 180^{o},270^{o}\}$