Александр sin(x) + sin(3x) = 4cos(x) sin(x) + 3sin(x) - 4sin(x) - 4 + 4sin(x) = 0 4sin(x) - 4sin(x) - 4sin(x) + 4 = 0 sin(x) = ±1 --> x = π/2 + πk, k - целое.