sin(2x+x)=sin2x.cosx+cos2x.sinx =2sinxcosx.cosx+(-2sin^2x)sinx =2sinxcos^2+sinx-2sin^3x =sinx(2cos^2x+1)-2sin^3x=sinx(2-2sin^2x+1)-2sin^3x =2sinx-2sin^3x+sinx-2sin^3x = 3sinx-4sin^3x