m^2=n+2 (m^2-n=2)n^2=m+2 (n^2-m=2)相减得m^2-n^2＝n-m变换得(m-n)(m+n+1)=0,因m不等于n，m+n+1＝0，m+n＝-1m^3-2mn+n^3=m(m^2-n)+n(n^2-m) =2m+2n =-2
可能是5吧
