m^2=n+2
n^2=m+2
相减：
m^2-n^2=n-m
(m-n)(n+m)=n-m
当m≠n时,m+n=-1
m^3+n^3-2mn
=m*m^2+n*n^2-2mn
=m(n+2)+n(m+2)-2mn
=mn+2m+mn+2n-2mn
=2(m+n)
=-2