xy2-2xy-3x-y2-2y-1
=（xy2-2xy-3x）-（y2+2y+1）
=x(y2-2y-3)-(y+1)2
=x(y+1)(y-3)-(y+1)平方
=(y+1)[x(y-3)-y-1]
=(y+1)(xy-3x-y-1)