2cos3x ≤ 1
cos3x ≤ \frac{1}{2} \frac{π}{3}+2πn ≤ 3x ≤ \frac{5π}{3}+2πn, n ∈ Z \frac{π}{9}+\frac{2π}{3}n ≤ x ≤ \frac{5π}{9}+\frac{2π}{3}n, n ∈ Z