Use trigonometric identifies to solve cos2 theta costheta

Use trigonometric identifies to solve cos(2 theta) - cos(theta) = 0 exactly for 0 lessthanorequalto theta

Cos(2theta) _ cos theta=0

Cos 2theta=2cos^2theta -1

2cos^2 theta -1-cos theta=0

2cos^2theta-cos theta -1=0

(2cos theta+1)(cos theta-1)=0

Cos theta=-1/2 and cos theta=1

Theta=2pi/3,4pi/3 and theta=0

Theta=0degree,120 degree ,240 degree