Problem Analysis #44

541.52s
33,459 toks
Problem Statement

Let f(x)f(x) be an increasing continuous function on the entire real line, and let f1(x)f^{-1}(x) be its inverse function. The function f1(x)f^{-1}(x) satisfies the following conditions:\begin{center}\fbox{\begin{minipage}{0.9\textwidth}(A) For x1|x| \leq 1,4(f1(x))2=x2(x25)2.4 \left(f^{-1}(x)\right)^2 = x^2(x^2 - 5)^2.(B) For x>1|x| > 1,f1(x)=ex1+1.\left|f^{-1}(x)\right| = e^{|x|-1} + 1.\end{minipage}}\end{center}For a real number mm, let g(m)g(m) denote the number of intersection points between the line of slope mm passing through (1,0)(1,0) and the curve y=f(x)y = f(x). Suppose that g(m)g(m) is discontinuous at m=am = a and m=bm = b with a<ba < b. Evaluateg(a)×(limma+g(m))+g(b)×(lnbb)2.g(a) \times \left(\lim_{m \to a^+} g(m)\right)+ g(b) \times \left( \frac{\ln b}{b} \right)^{2}.(Here, you may use limxlnxx=0\displaystyle\lim_{x \to \infty} \dfrac{\ln x}{x} = 0.) [4 points]\

Ground Truth
11
Execution Trace
Reasoning Process

[THOUGHT] I will:

  1. Define the function F(x)=f^{-1}(x) piecewise in Python.
  2. Solve analytically (via sympy) for tangency points using φ(t)=F(t)−tF’(t)−1=0 in each region.
  3. Obtain corresponding slopes s=F’(t) and m=1/s.
  4. Numerically determine g(m) (number of intersections) as function of m using root counts of F(t)−1−t/m=0, scanning m and detecting where it changes to confirm the discontinuity points a<b and the associated g-values and one-sided limits.
  5. Compute the requested expression.

I'll start by implementing F and solving φ(t)=0 piecewise, and then scan m to see g(m) behavior. [/THOUGHT] [PYTHON] import sympy as sp import mpmath as mp

t = sp.symbols('t', real=True)

Define F and F' in each region symbolically

F_left = -(sp.exp(-t-1) + 1) # t < -1 Fmid = sp.Rational(1,2)t(5 - t**2) # -1 <= t <= 1 F_right = sp.exp(t-1) + 1 # t > 1

F_left_prime = sp.diff(F_left, t) Fmid_prime = sp.diff(Fmid, t) F_right_prime = sp.diff(F_right, t)

F_left, F_left_prime, Fmid, Fmid_prime, F_right, F_right_prime [/PYTHON]

Tool Output
[PYTHON OUTPUT] (-exp(-t - 1) - 1, exp(-t - 1), t*(5 - t**2)/2, 5/2 - 3*t**2/2, exp(t - 1) + 1, exp(t - 1)) [/PYTHON OUTPUT]
Reasoning Process

[THOUGHT] From the given conditions we obtain the inverse function [ f^{-1}(t)=F(t)= \begin{cases} -(e^{-t-1}+1), & t<-1,\[3pt] \frac{t(5-t^2)}{2}, & -1\le t\le 1,\[3pt] e^{t-1}+1, & t>1. \end{cases} ]

Let the line through ((1,0)) of slope (m) be (y=m(x-1)).
Intersections with (y=f(x)) are equivalent, in ((t,F(t)))-space, to intersections of [ F(t) = 1+\frac{t}{m}. ] That is, intersections between the curve (y=F(t)) and the line (y = 1+kt) with slope (k=\frac1m).

Counting intersections as a function of (m) (or (k)) uses that (F) is increasing and (F'(t)\ge1) everywhere, with equality only at (t=-1,1). A careful analysis (using sample slopes and monotonicity/concavity) shows:

  • For (m<0): exactly 1 intersection.
  • For (m=0): exactly 1 intersection.
  • For (0<m<b): exactly 3 intersections.
  • For (m=b): exactly 2 intersections (one is a tangency on the left exponential branch).
  • For (b<m): exactly 1 intersection.

So (g(m)) has jump discontinuities at (m=0) and at (m=b). Hence [ a=0,\quad b>0. ]

To find (b), use the tangency on the left branch (x\le -2). In original ((x,y))-space, for (x\le -2), [ f(x) = -1-\ln(-x-1),\quad f'(x) = -\frac1{x+1}. ] Tangency of (y = m(x-1)) to this branch at (x=s\le -2) requires [ m = f'(s) = -\frac1{s+1},\qquad f(s) = m(s-1). ] From (m = -1/(s+1)) we have (s+1 = -1/m), so (-s-1 = 1/m). Then [ f(s) = -1-\ln(-s-1) = -1-\ln\frac1m = -1+\ln m. ] On the other hand, [ m(s-1) = m\Big(-\frac1m -1 -1\Big) = -1-2m. ] Equating gives [ -1+\ln m = -1-2m \quad\Longrightarrow\quad \ln m = -2m. ] Thus (b) is the positive solution of (\ln b = -2b), so [ \frac{\ln b}{b} = -2 \quad\Longrightarrow\quad \left(\frac{\ln b}{b}\right)^2 = 4. ]

Now the required data:

  • At (a=0): (g(0)=1), and (\displaystyle\lim_{m\to 0^+}g(m)=3).
  • At (b): (g(b)=2), and (\left(\dfrac{\ln b}{b}\right)^2 = 4).

Therefore, [ g(a)\cdot\left(\lim_{m\to a^+} g(m)\right)

  • g(b)\cdot\left(\frac{\ln b}{b}\right)^2 = 1\cdot 3 + 2\cdot 4 = 3 + 8 = 11. ]

\boxed{11}

Final Model Output
11

Comparative Analysis (P44) (Average across runs)

Avg Time Taken

Deepseek-V3.2
975.6
Grok-4.1-fast
593.1
GPT-5.1 (high)
541.5
Claude-Opus-4.5
411.3
Llama-VARCO-8B-Instruct
390.9
Gemini-3-Pro-Preview
196.4
Solar-Pro-2 (31B)(high)
157.7
A.X-4.0 (72B)
39.7
HCX-007(high)
25.3
EXAONE-4.0.1-32B (high)
24.1

Avg Token Usage

Claude-Opus-4.5
111449.0
Grok-4.1-fast
96753.0
GPT-5.1 (high)
33459.0
Deepseek-V3.2
29214.0
Gemini-3-Pro-Preview
22016.0
Llama-VARCO-8B-Instruct
20708.0
Solar-Pro-2 (31B)(high)
18795.0
EXAONE-4.0.1-32B (high)
9373.0
HCX-007(high)
3907.0
A.X-4.0 (72B)
3217.0