3. Решить уравнение: 4) cos 5x + cos 3x = 0

2 cos((5x+3x)/2) cos((5x-3x)/2) = 0

2 cos(4x) cos(x) = 0

cos(4x) = 0 или cos(x) = 0. 4x = π/2 + πn, x = π/8 + πn/4. x = π/2 + πn, n ∈ Z