Question

A quadratic polynomial with real coefficients and leading coefficient $1$ is called disrespectful if the equation $p(p(x)) = 0$ is satisfied by exactly three real numbers. Among all the disrespectful quadratic polynomials, there is a unique such polynomial $\tilde{p}(x)$ for which the sum of the roots is maximized. What is $\tilde{p}(1)?$

$\dfrac{5}{16}$

$\dfrac{1}{2}$

$\dfrac{5}{8}$

$1$

$\dfrac{9}{8}$

Answer: A

Concepts:polynomial quadratic optimization

Difficulty rating: 2380

Solution:

Let $p$ have roots $r$ and $s.$ Then $p(p(x)) = 0$ splits into $p(x) = r$ and $p(x) = s,$ with discriminants $(r - s)^2 + 4r$ and $(r - s)^2 + 4s.$ Exactly three real roots means one discriminant is $0$ and the other positive.

Take $(r - s)^2 + 4s = 0$ and set $u = r - s.$ Then $s = -\tfrac{u^2}{4}$ and the other discriminant is $4u,$ so $u>0.$ Also $r+s=-\tfrac{u^2}{2}+u,$ maximized at $u=1,$ giving $r=\tfrac34$ and $s=-\tfrac14.$

So $\tilde{p}(x) = \left(x - \tfrac34\right)\left(x + \tfrac14\right),$ and $\tilde{p}(1) = \tfrac14\cdot\tfrac54 = \tfrac{5}{16}.$

Thus, the correct answer is A.

2021 AMC 12A Fall Problem 23

Answer: A

Difficulty rating: 2380

Solution:

Problem 23 in Other Years