4 cos³x = sin(х-3π/2)
4 cos³x = -sin(3π/2 - х)
4 cos³x = сosх
cos²x = 1/4
1) cosx = 1/2
х₁ = ±π/3 + 2πn n∈Z
2) cosx = -1/2
х₂ = ±2π/3 + 2πn n∈Z
