Chapter 6 - Section 6.3 - Trigonometric Equations Involving Multiple Angles - 6.3 Problem Set - Page 340: 20

$\theta=\{60^o+120^ok\;\;\}$

$cos(3\theta)=-1$ $3\theta=cos^{-1}(-1)$ The period of the cosine function is $360^o$ $3\theta=180^o+360^ok\;\;\;\;\;\;\;\;\;\;\;\;\;\; $ $\theta=60^o+120^ok\;\;\;\;\;\;\;\;\;\;\;\;\;\; $ $\theta=\{60^o+120^ok\;\;\},\;\;\;\;\;\;\;\;\;\;when \;\;\;K=\{0,1,2,3\}$