## Precalculus (6th Edition)

$\{0^{o},180^{o}\}$
$\cos^{2}\theta-\sin^{2}\theta =1$ ... recognize a double angle (cosine) identity $\cos 2\theta=1$ If $0 \leq \theta < 360^{o}\quad/\times 2$ $0 \leq 2\theta < 720^{o}$ $2\theta=0^{o}\quad$ or $\quad 2\theta=360^{o}$ $\theta=0^{o}\quad$ or $\quad \theta=180^{o}$ Solution set= $\{0^{o},180^{o}\}$