Solve (cos x)(cos 2x) = 0 for the interval [0, ππ]
x = π/3, x = π/2
x = π/3, x = π/4, x = π/6
x = π/4, x = π/2, x = 3π/4
x = 3 π/4, x = π

Hello :
(cos x)(cos 2x) = 0

(cos x= 0)0r(cos 2x =0) 
sur :  [0, π] 
1) cos x= 0  : x = π/2 
2 ) cos 2x =0 : x = π/4 or x=3π/4
conclusion : x = π/4, x = π/2, x = 3π/4