12.Let f(x) = x^2 + 6x + c for all real numbers x,where c is some real number.For what values of c does f(f(x)) have exa
问题描述：
12.Let f(x) = x^2 + 6x + c for all real numbers x,where c is some real number.For what values of c does f(f(x)) have exactly 3 distinct real roots?
答
Suppose f has only one distinct root r1.Then,if x1 is a root of f(f(x)),it must be the case that f(x1) = r1.As a result,f(f(x)) would have at most two roots,thus not satisfying the problem condition.H...