(x-y)的2次方-2(x-y)+(x-y)-5(x-y)
=(x-y)²-2(x-y)+(x-y)-5(x-y)
=(x-y)²-(x-y)-5(x-y)
=(x-y)²-6(x-y)
=(x-y)(x-y-6)-(x的2次方y+3xy-4）+3（x的2次方y-xy+2）