xy-2x+6y-12 xy+8
=xy-12 xy-2x+6y+8
=-11xy-2x+6y+8
或者
xy-2x+6y-12
=(xy-2x)+(6y-12)
=x(y-2)+6(y-2)
=(x+6)(y-2)
那么 xy+8是干什么的?