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
Перенесём косинус влево
1=2*cos(4x)
cos(4x)=1/2
4x=2*Pi*n+-Pi/3
x=Pi/2+-Pi/12=1/12(6*Pi*n+-Pi)
22/7 меньше чем 3,14159263558