Помогите решить неравенства 36cos 9x ≥18
Cos 9x ≥ 1/2 -π/3 +2πk ≤ 9x ≤ π/3 + 2πk , k ∈Z -π/27 + 2πk/9 ≤ x ≤ π/27 + 2πk/9 , k ∈Z