因为m^2=n+2,n^2=m+2
所以 m^2-n^2=（n+4）-(m+4)=n-m
即 (m-n)(m+n)=-(m-n)
m+n=-1
m^3-2mn+n^3=m·m^2-2mn+n·n^2
=m(n+4)-2mn+n(m+4)
=mn+4m-2mn+mn+4n
=4(m+n)
因为m+n=-1
所以m^3-2mn+n^3=-4