x^7-x^5y^2+x^2y^5-7y,是不是y^7
如果是的话
==>x^5(x^2-y^2)+y^5(x^2-y^2)
==>(x^2-y^2)*(x^5+y^5)
==>(x+y)(x-y)(x^5+y^5)