Sin^2*3x-cos^2*3x<или=-корень из 3/2
-cos6x≤√3/2 cos6x≥-√3/2 7π/6+2πn≤6x≤17π+2πn 7π/36+πn/3≤x≤17π/36+πn/3 x∈[7π/36+πn/3;17π/36+πn/3]