Cos 4x - cos 2x = 0
find all solutions to the equation in the interval [0, 2ð).
ð, pi/3 2pi/3 4pi/3 5pi/3, 0?

cos 4 x - cos 2 x = 0
2cos^2 2x-1 - cos 2x = 0
2u^2 -u -1 = 0 (u-1)(2u+1) = 0
u = 1; u = -1/2
cos 2x = 1 ;
cos 2x = -1/2
2x = 0 ;
 2x = pi-pi/3,
2x = pi + pi/3
x = 0 + npi; x = pi/3 + npi, x = 2pi/3 + npi
x = 0, pi; x = pi/3, 4pi/3,
x = 2pi/3, 5pi/3 \
so solutions will be
 x = 0, pi/3, 2pi/3, 4pi/3, 5pi/3, pi
hope it helps