回复：谢谢您给我打气,

From "5 is the only root of f(f(x))" to "f(x)= a(x-5)^5 + b" is sup cool. But only genius could jump to there immediately. For general people, like me, the natual conclusion is

f(f(x)) = a(x-5)^2(x^2+bx+c). or a(x-5)^4.