Помогите: 10 cos² x + 3 cos x - 1 = 0

Question

Помогите: 10 cos² x + 3 cos x - 1 = 0

Answer 1

10 Cos² x + 3 Cos x -1 = 0
t = Cos x, |t| ≤ 1

10t² +3t -1 = 0
D = 9 +40 = 49
t = (-3 -7)/20 = -1/2
t = (-3+7)/20 = 1/5

Cos x = -1/2
x = ± 2π/3 +2πn, n ∈ Z

Cos x = 1/5
x = ± arccos(1/5) +2πn, n ∈ Z