3. Решить уравнение: 2) 10 cos²x + 3 cos x = 1;

10 cos²x + 3 cos x - 1 = 0

Пусть y = cos x. 10y² + 3y - 1 = 0. y = (-3 ± √9 + 40)/20 = (-3 ± 7)/20. y₁ = 4/20 = 1/5, y₂ = -10/20 = -1/2.

cos x = 1/5 или cos x = -1/2. x = ±arccos(1/5) + 2πn, x = ±2π/3 + 2πn, n ∈ Z