已知(m+n)x^n·y^m-2·(3xy^2+5x^2·y)=21x^m·y^n+1+35x^m+1·y^n,求m和n
问题描述：
已知(m+n)x^n·y^m-2·(3xy^2+5x^2·y)=21x^m·y^n+1+35x^m+1·y^n,求m和n
答
(m+n)x^n·y^m-2·(3xy^2+5x^2·y)=21x^m·y^n+1+35x^m+1·y^n
3(m+n)x^n+1·y^m+5(m+n)x^m+1·y^n=21x^m·y^n+1+35x^m+1·y^n
同类项系数相等
所以3(m+n)=21
5(m+n)=35
m+n=7
求不出具体的m和n