原式=(x-y)^4+x(x-y)^3+y(x-y)^3
=(x-y)^4+(x+y)(x-y)^3
=(x-y)^3((x-y)+(x+y))
=2x(x-y)^3=(x-y)^4+(x+y)(x-y)^3怎么得出来的。。。x(x-y)^3+y(x-y)^3有公因式（x-y)^3,提公因式嘛