单项式-5x^4m-ny2与x^2y^m+n/5的和仍是单项式
4m-n=2
(m+n)/5=2
这两个单项式的积
=(-5x^4m-ny2)*(x^2y^m+n/5)
=-5x^(2+2)y(2+2)
=-5x^4y^4