没有公因式
用平方差
4(m+n)^2-9(m-n)^2
=[2(m+n)]^2-[3(m-n)]^2
=(2m+2n)^2-(3m-3n)^2
=(2m+2n-3m+3n)(2m+2n+3m-3n)
=5m*(-n)
=-5mn