计算(9m^2/-3m-n)+n^2/n+3m 一定要
问题描述：
计算(9m^2/-3m-n)+n^2/n+3m 一定要
答
9m^2/-3m-n+n^2/n+3m
=-9m^2/n+3m +n^2/n+3m
=(n^2-9m^2)/n+3m
=(n+3m)(n-3m)/n+3m
=n-3m