## Trigonometry 7th Edition

$\theta=\{60^o,180^o,300^o\}$
$cos(3\theta)=-1$ $3\theta=cos^{-1}(-1)$ We know $cos(\theta)$ is negative in quardent $II$ and quardent $III$ The period of the cosine function is $360^o$ $3\theta=180^o\;\;\;\;or\;\;\;\;\;\;3\theta=360^o+180^o\;\;\;\;or\;\;\;\;\;\;3\theta=720^o+180^o\;\;\;\;\;\;\;\;\;\;$ $\theta=60^o\;\;\;\;or\;\;\;\;\;\;\theta=180\;\;\;\;or\;\;\;\;\;\;\theta=300^o\;\;\;\;\;\;\;\;\;\;\;\;\;$ $\theta=\{60^o,180^o,300^o\}$