m2=m+2
m3=m(n+2)=mn+2m
n2=m+2
n3=n(m+2)=mn+2n
m2-n2=(n+2)-(m+2)
(m+n)(m-n)=-(m-n)
m≠n则m-n≠0
所以m+n=-1
原式=mn+2m-2mn+mn+2n
=2(m+n)
=2×(-1)
=-2