f(m)=f(n),
m^2-2008m=n^2-2008n
m^2-n^2=2008m-2008n
(m+n)(m-n)=2008(m-n)
m-n不等于0
m+n=2008
f(m+n)=f(2008)=2008^2-2008*2008=0