﹣m二次方n(x-y)n次方+mn二次方（x-y）n+1次方
=mn(x－y)^n*n(x－y)﹣mn(x－y)^n*m
=mn(x－y)^n[n(x－y)－m]
=mn(x－y)^n(nx－ny－m)
如有不懂之处还可问我!