m^2=n+2,n^2=m+2(m不等于n),求m^3-2mn+n^3的值
问题描述：
m^2=n+2,n^2=m+2(m不等于n),求m^3-2mn+n^3的值
因为m^2=n+2,n^2=m+2 所以 m^2-n^2=n-m 即 (m-n)(m+n)=n-m m+n=1
为什么得m+n=1,求详解.
答
(m-n)(m+n)=n-m
(m-n)(m+n+1)=0
m+n=-1
m^2=n+2,n^2=m+2
m^2+n^2=n+2+m+2
(m+n)^2-2mn=4-1
1-2mn=3
mn=-1
m^3-2mn+n^3=（m+n)^3-3m^2n-3n^2m-2mn=-1-3mn(m+n)-2mn=-1+mn=-2