Срешением ! sin3x+корень из 3 cos3x < 0. ()

2*(1/2*sin3x+√3/2*cos3x)<0
2*sin(3x+π/3)<0
sin(3x+π/3)<0
π+2πn<3x+π/3<2π+2πn
2π/3+2πn<3x<5π/3+2πn
5π/9+2πn/3<x<5π/9+2πn/3
x∈(5π/9+2πn/3;5π/9+2πn/3,n∈z)