因式分解：(xy+1)(x+1)(y+1)+xy

(xy+1)(x+1)(y+1)+xy
展开(x+1)(y+1)展开,得
(xy+1)(xy+x+y+1)+xy
即(xy+1)(xy+1+x+y)+xy
将(xy+1)当做一个整体,展开得
(xy+1)^2+(xy+1)(x+y)+xy
十字相乘法,得
(xy+x+1)(xy+y+1)
答案：(xy+1)(x+1)(y+1)+xy = (xy+x+1)(xy+y+1)