2(m²+n²)(m+n)-(m²-n²)²
=2(m²+n²)(m+n)-(m+n)²(m-n)²
=(m+n)[2(m²+n²)-(m+n)(m-n)²]
是否m+n后面缺个平方?是缺了平方：2(m^2+n^2)(m+n)^2-(m^2-n^2)^2