（3m+3n+1(3m+3n-1)=80,求M+N的值
问题描述：
（3m+3n+1(3m+3n-1)=80,求M+N的值
答
（3m+3n+1(3m+3n-1)=80
(3m+3n)^2-1^2 = 80
(3m+3n)^2 = 80 + 1 = 81
---> [3(m+n)]^2 = 81
(m+n)^2 = 9
m+n = 3 或者m + n = -3