Решить уравнение cos2x-2cosx=0
Cos2x = 2cos^2x - 1 2cos^2x - 2cosx = 0 cosx(2cosx - 2) = 0 cosx = 0 либо cosx = 1 x = π/2 + πn, n ∈ Z, x = 2πk, k ∈ Z