Math Problem Statement

다항함수 f(x)가 다음 조건을 만족시킬 때, f'(5)의 값은? (1) \( \lim_{x \to \infty} \frac{f(x) - 2x^2}{x^2 - 1} = 2 \) (2) \( \lim_{x \to 1} \frac{f(x) - 2x^2}{x^2 - 1} = 2 \)

Solution

The problem is asking for the value of f(5)f'(5), where the function f(x)f(x) satisfies the given limit conditions:

  1. limxf(x)2x2x21=2\lim_{x \to \infty} \frac{f(x) - 2x^2}{x^2 - 1} = 2
  2. limx1f(x)2x2x21=2\lim_{x \to 1} \frac{f(x) - 2x^2}{x^2 - 1} = 2

Step 1: Analyze the first limit (as xx \to \infty)

We are given: limxf(x)2x2x21=2\lim_{x \to \infty} \frac{f(x) - 2x^2}{x^2 - 1} = 2

This suggests that f(x)f(x) behaves like 2x22x^2 as xx \to \infty, but we need to account for any potential lower-degree terms. Let's assume f(x)f(x) takes the form: f(x)=2x2+ax+bf(x) = 2x^2 + ax + b We now substitute this into the limit expression: (2x2+ax+b)2x2x21=ax+bx21\frac{(2x^2 + ax + b) - 2x^2}{x^2 - 1} = \frac{ax + b}{x^2 - 1} As xx \to \infty, this simplifies to: ax+bx2ax+bx2\frac{ax + b}{x^2} \approx \frac{a}{x} + \frac{b}{x^2} Since the limit is 2, we must have a=0a = 0 (as the leading term would otherwise blow up), meaning: f(x)=2x2+bf(x) = 2x^2 + b Thus, the first limit tells us that f(x)=2x2+bf(x) = 2x^2 + b.

Step 2: Analyze the second limit (as x1x \to 1)

We are given: limx1f(x)2x2x21=2\lim_{x \to 1} \frac{f(x) - 2x^2}{x^2 - 1} = 2 Substitute f(x)=2x2+bf(x) = 2x^2 + b into the expression: (2x2+b)2x2x21=bx21\frac{(2x^2 + b) - 2x^2}{x^2 - 1} = \frac{b}{x^2 - 1} As x1x \to 1, the denominator becomes 0, so for the limit to exist, we use the fact that x21=(x1)(x+1)x^2 - 1 = (x-1)(x+1). Thus: limx1b(x1)(x+1)=2\lim_{x \to 1} \frac{b}{(x-1)(x+1)} = 2 Evaluating this limit as x1x \to 1 gives: b2=2b=4\frac{b}{2} = 2 \quad \Rightarrow \quad b = 4

Step 3: Derive f(x)f'(x) and find f(5)f'(5)

From the above, we have f(x)=2x2+4f(x) = 2x^2 + 4. Taking the derivative: f(x)=4xf'(x) = 4x Thus: f(5)=4×5=20f'(5) = 4 \times 5 = 20

The value of f(5)f'(5) is 20\boxed{20}.


Would you like more details, or do you have any further questions?

Here are five related questions for further understanding:

  1. How do limits help determine the form of a function?
  2. What happens if the limits were different at infinity and near a specific point?
  3. Why is it necessary to account for lower-order terms in function approximations?
  4. How does the derivative change with different functional forms?
  5. Can you explain how to apply L'Hopital's Rule to limits involving indeterminate forms?

Tip: When working with limits, always look for patterns or terms that dominate the behavior of the function as xx \to \infty or near specific points.

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Math Problem Analysis

Mathematical Concepts

Limits
Polynomials
Differentiation

Formulas

lim_{x -> infinity} (f(x) - 2x^2) / (x^2 - 1) = 2
lim_{x -> 1} (f(x) - 2x^2) / (x^2 - 1) = 2
f(x) = 2x^2 + b
Derivative: f'(x) = 4x

Theorems

Limit definition
Polynomial approximation
Derivative rules

Suitable Grade Level

University-level calculus