Question 751524
cos2x + cos x
replace cos 2x with 2cos^2-1
2cos^2-1 + cos x = 0
Rearrange
2cos^2x + cos x - 1 = 0
(2cosx - 1)(cosx + 1) = 0
 cosx = 1/2 or -1
 
x = 60 degs, 300degs  and 180degs

  = pi/3 rads, 5pi/3 rads and pi