x减y的m次方乘以y减x的2m次方加y减x的3m次方
=x-y^m*y-x^(2m)+y-x^(3m)
=x-y^(m+1)-x^(2m)+y-x^(3m)
=x-x^(2m)-x^(3m)+y-y^(m+1)