sin3x+sinx=sin2x
2sin((3x+x)/2)*cos((3x-x)/2)=sin(2x)
2sin(2x)*cos(x)=sin(2x)
2sin(2x)*cos(x)-sin(2x)=0
sin(2x)*(2cos(x)-1)=0
1) sin(2x)=0 => 2x=pi*n => x=pi*n/2
2) 2cos(x)-1=0 => 2cos(x)=1 => cos(x)=1/2 => x=±pi/3+2*pi*n
