f(b)=b ==> b^2/(ab-2)=b ==> b=ab-2 ==> (a-1)b=2
==> a=2,b=2或a=3,b=1
当a=3,b=1时,f(x)=x^2/(3x-2),f(-b)=f(-1)=(-1)^2/(-5)=-1/5