回复：Math is not riddle

f(0)=-128/9, as you did. It is my carelessness....

I think pfollowing is the explanation: a(x-5)^2+b,...a=-b/9...

ff=-b/9(-b/9(x-5)^2+b-5)^2+b=0,only b=2,x=5 is unique real root, b=8,has three 3 roots,not good.key point b-5 <0. Here is not riddle.