十字相乘法?
xy²+2xy-3x-y²+2y-1
=（x-1）y^2+2(x+1)y-(3x+1)
十字相乘
1 -1
x-1 3x+1
=（y-1）[(x-1)y-(3x+1)]
=(y-1)(xy-3x-y-1)