sin(2x+y)=sin(x+y+x)=sin(x+y)*cosx+cos(x+y)*sinxsiny=sin(x+y-x)=sin(x+y)*cosx-cos(x+y)*sinx所以,sin(x+y)cosx-1/2[sin(2x+y)-siny]=sin(x+y)*cosx-cos(x+y)*sinx=sin(x+y-x)=siny