n^2/m+m^2/n
=(n^3+m^3)/mn （因为m的平方减去n的平方等于mn ）
=（n+m)(n^2-mn+m^2)/mn
=(n+m)(n^2-m^2+n^2+m^2)/mn
=(n+m)*2n^2/(m+n)(m-n)
=2n^2/(m-n)
=2(m^2-mn)/(m-n)
=2m