Cos x - 2 sin x/4 * cos x/4 = 0

Cosx - 2sin (x/4) cos (x/4) = 0

cosx - sin (x/2) = 0

1 - 2sin² (x/2) - sin (x/2) = 0

2sin² (x/2) + sin (x/2) - 1 = 0

sin (x/2) = t

2t² + t - 1 = 0

D = 1 + 4*2*1 = 9

t1 = (- 1 - 3) / 4

t1 = - 1

t2 = (- 1 + 3) / 4

t2 = 1/2

1) sin (x/2) = - 1

x/2 = - π/2 + 2πk, k∈Z

x1 = - π + 4πk, k∈Z

2) sin (x/2) = 1/2

x/2 = (-1) ^ (n) * arcsin (1/2) + πn, n∈Z

x/2 = (-1) ^ (n) * (π/6) + πn, n∈Z

x2 = (-1) ^ (n) * (π/3) + 2πn, n∈Z