6 - cos²x - 5cosx + 5 = 0
-6cos²x - 5cosx + 11 = 0
пусть cosx = a
-6a² - 5a + 11 = 0
D = (-5)² - 4 * (-6) * 11 = 25 + 264 = 289
а₁ = (5 + √289) / (-12) = (5 + 17) / (-12) = -11/6
а₂ = (5 - √289) / (-12) = (5 - 17) / (-12) = (-12) / (-12) = 1
так как -11/6 ≤ -1, число не подходит
cosx = 1
x = 2πn, n ∈ Z