2x+(π/9)=±arccos(-√3/2)+2πk, k ∈ Z;
2x+(π/9)=±(π-arccos√3/2)+2πk, k ∈ Z;
2x=-(π/9)±(π-(π/6))+2πk, k ∈ Z;
2x=-(π/9)±(5π/6)+2πk, k ∈ Z;
x=-(π/18)±(5π/12)+πk, k ∈ Z;
или
х₁ =-(π/18)+(5π/12)+πk, k ∈ Z;
х₁ =(13π/36)+πk, k ∈ Z;
х₂=-(π/18)-(5π/12)+πn, n ∈ Z;
х₂=-(17π/36)+πn, n ∈ Z.
О т в е т. (13π/36)+πk, k ∈ Z;-(17π/36)+πn, n ∈ Z.