2025 AMC 12A Problems/Problem 25: Difference between revisions

Revision as of 04:49, 7 November 2025

Polynomials $P(x)$ and $Q(x)$ each have degree $3$ and leading coefficient $1$ , and their roots are all elements of $\{1,2,3,4,5\}$ . The function $f(x) = \tfrac{P(x)}{Q(x)}$ has the property that there exist real numbers $a < b < c < d$ such that the set of all real numbers $x$ such that $f(x) \leq 0$ consists of the closed interval $[a,b]$ together with the open interval $(c,d)$ . How many functions $f(x)$ are possible?

Solution 1

Step 1: Sign Chart Analysis

From the given $f(x) \le 0$ on $[a,b] \cup (c,d)$ and $f(x) > 0$ elsewhere, we deduce:

$a$ and $b$ are zeros of $f$ since we transition from $f > 0$ to $f \le 0$ at $a$ and from $f \le 0$ to $f > 0$ at $b$ .
$c$ and $d$ are vertical asymptotes or holes of $f$ since $f(x)$ is undefined at $c$ and $d$ . We have $f \le 0$ in $(c,d)$ , and $f> 0$ outside the interval.

Thus the sign pattern is:

$\textstyle \begin{array}{cccccccccc} & (-\infty, a) & a & (a,b) & b & (b,c) & c & (c,d) & d & (d,\infty) \\ f(x) & + & 0 & - & 0 & + & \text{pole} & - & \text{pole} & + \end{array}$

Step 2: Structure of $f(x)$

According to the given conditions, $ f(x) $ can be expressed as: $f(x) = \frac{(x-p_1)(x-p_2)(x-p_3)}{(x-q_1)(x-q_2)(x-q_3)}.$ Combining the sign analysis from Step 1, we can rewrite this expression as: $f(x) = \frac{(x-a)(x-b)(x-p_3)}{(x-c)(x-d)(x-q_3)}.$ Furthermore, we can break down the expression into two parts: $f(x) = \frac{(x-a)(x-b)}{(x-c)(x-d)} \cdot \frac{x-p_3}{x-q_3}.$ Defining $ f(x) = f_1(x) \cdot f_2(x) $, we have: $f_1(x) = \frac{(x-a)(x-b)}{(x-c)(x-d)}, \quad f_2(x) = \frac{x-p_3}{x-q_3}.$ Step 3: Analysis of $ p_3 $ and $ q_3 $

$ f_1(x) $ completely satisfies the sign chart for $ f(x) $. We can conclude that $ f_2(x) $ must be positive in every interval; otherwise, $ f_2(x) $ would be negative in interval $( p_3, q_3 )$ , which introduces extra sign changes. This forces the condition:$ p_3 = q_3 $. Let $ p_3 = q_3 = t $.
Furthermore, $ t $ cannot lie within $ [a,b] \cup (c,d) $. If it did, it would contradict the given condition that the set $ \{x : f(x) \le 0\} $ consists of the closed interval $ [a, b] $ and the open interval $ (c, d) $.
Additionally, $ t $ cannot equal to $ a $ or $ b $. If it did equal, that point would be a hole (a point of discontinuity) in the graph of $ f(x) $, which contradicts the closed interval $ [a, b] $.

Thus, $ t $ can only equal to $ c $ or $ d $, or lie in an interval outside of $ [a,b] \cup (c,d) $.

Step 4: Ways to choose $a,b,c,d$ from $\{1,2,3,4,5\}$

Given $a < b < c < d$ and the condition that $\{x: f(x) \leq 0\} = [a,b] \cup (c,d)$ , the number of ways to choose $a,b,c,d$ from $\{1,2,3,4,5\}$ is: $\binom{5}{4} = 5$ Step 5: The possible value of $a$ , $b$ , $c$ , $d$ , and $t$ for the five cases are:

Case 1: $[1,2]\cup (3,4)$ , $t$ can be 3, 4, or 5
Case 2: $[1,2]\cup (3,5)$ , $t$ can be 3 or 5
Case 3: $[1,2]\cup (4,5)$ , $t$ can be 3, 4, or 5.
Case 4: $[1,3]\cup (4,5)$ , $t$ can be 4 or 5.
Case 5: $[2,3]\cup (4,5)$ , $t$ can be 1, 4, or 5.

Summing up all possible cases, $3 + 2 + 3 + 2 + 3 = 13$ possible functions $f(x)$ .

Thus the answer is $\boxed{E}$ .

- Victor Zhang

Video Solution 1 by OmegaLearn

https://youtu.be/SPbTyq3Dz_0

@@ Line 7: / Line 7: @@
 From the given <imath>f(x) \le 0</imath> on <imath>[a,b] \cup (c,d)</imath> and <imath>f(x) > 0</imath> elsewhere, we deduce:
 * <imath>a</imath> and <imath>b</imath> are zeros of <imath>f</imath> since we transition from <imath>f > 0</imath> to <imath>f \le 0</imath> at <imath>a</imath> and from <imath>f \le 0</imath> to <imath>f > 0</imath> at <imath>b</imath>.
-* <imath>c</imath> and <imath>d</imath> are vertical asymptotes or holes of <imath>f</imath> since <imath>f(x)</imath> is undefined at <imath>c</imath> and <imath>d</imath>. We have <imath>f \le 0</imath> in <imath>(c,d)</imath> , and  <imath>f > 0</imath> outside the interval <imath>f > 0</imath>.
+* <imath>c</imath> and <imath>d</imath> are vertical asymptotes or holes of <imath>f</imath> since <imath>f(x)</imath> is undefined at <imath>c</imath> and <imath>d</imath>. We have <imath>f \le 0</imath> in <imath>(c,d)</imath> , and  <imath>f> 0</imath> outside the interval.
 Thus the sign pattern is:
@@ Line 38: / Line 38: @@
 Given <imath>a < b < c < d</imath> and the condition that <imath>\{x: f(x) \leq 0\} = [a,b] \cup (c,d)</imath>, the number of ways to choose <imath>a,b,c,d</imath> from <imath>\{1,2,3,4,5\}</imath> is:
 <cmath>\binom{5}{4} = 5 </cmath>
 Step 5: The possible value of <imath>a</imath>, <imath>b</imath>, <imath>c</imath>, <imath>d</imath>, and <imath>t</imath> for the five cases are:

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2025 AMC 12A Problems/Problem 25: Difference between revisions

Revision as of 04:49, 7 November 2025

Solution 1

Video Solution 1 by OmegaLearn