[tex]cos \frac{x}{3} \leq \frac{ \sqrt{3} }{2} [/tex]

cosx/3 ≤ √3/2
π/6 ≤ x/3 ≤ 2π -π/6;
π/6 ≤ x/3 ≤ 11π/6;
2π*k+ π/6 ≤ x/3 ≤ 11π/6  + 2π*k ;
6π*k+ π/2 ≤  x  ≤ 11π/2  + 6π*k ; K∈Z

совокупность отрезов вида x∈ [ 6π*k+ π/2   ; 11π/2  + 6π*k]  ; K∈Z