2 - 2cos^2 (x) - 5cos(x) + 1 = 0

2cos^2 (x) + 5cos(x) - 3 = 0

cos(x) = 1/2 cos(x) = -3

x = π/3 + 2πk нет решений

x = -π/3 + 2πk

Ответ : π/3 + 2πk , -π/3 + 2πk ; k € Z