Обозначим cos x = у и получим уравнение
6у² + у - 1 =0
D = 1² - 4·6·(-1) = 25; √25 = 5
y1 = (-1 - 5)/(2·6) = -6/12 = -1/2
y2 = (-1 + 5)/(2·6) = 4/12 = 1/3
cos x = -1/2
x = +-arccos(-1/2) + 2πn, n ∈ Z
x = +- 2π/3 + 2πn, n ∈ Z
cos x = 1/3
x = +- arccos(1/3) + 2πn, n ∈ Z