posted by Manny .
Find two standard position primary angles in radians by solving for the unknown. "Primary angles" are those angles which exist between 0 and 2pi. As usual, use exact values in your calculations.
cosx - 2cos^2x = 0
cosx(1 - 2cosx)=0
cosx=0 or cosx=1/2
x = pi/2, 3pi/2, pi/3, 5pi/3 (90,270,60,300º)