(3m+n)²-(m+3n)
=9m²+6mn+n²-m-3n
题目是否少了一个平方?
(3m+n)²-(m+3n)²
=(3m+n+m+3n)(3m+n-m-3n)
=(4m+4n)(2m-2n)
=8(m+n)(m-n)
供参考