заменим пока (3x -2 pi/3) на A
SinA ≥ 1/2
A ≥ π/6 +2πn 3x -2 pi/3 ≥ π/6 +2πn, x ≥ 5π/18 +2πn/3
A ≤ 5π/6 + 2πn 3x -2 pi/3 ≤ 5π/6 + 2πn, x ≤ 9π/18 +2πn/3
5π/18 +2πn/3 ≤ x ≤ 9π/18 +2πn/3
5π/18 +2πn/3 ≤ x ≤ π/2 +2πn/3
