Math Problem Statement

Evaluate the limit: lim (x -> infinity) (1 + 5 / (2x^2 - 3x))^(4x^2).

Solution

The problem in the image is asking to evaluate the limit:

limx(1+52x23x)4x2.\lim_{x \to \infty} \left( 1 + \frac{5}{2x^2 - 3x} \right)^{4x^2}.

This is a classic case involving limits of the exponential type, where the base approaches 11 and the exponent approaches infinity. Let us carefully evaluate this step by step.


Step 1: Rewrite the expression

Set: y=(1+52x23x)4x2.y = \left( 1 + \frac{5}{2x^2 - 3x} \right)^{4x^2}. Take the natural logarithm: lny=4x2ln(1+52x23x).\ln y = 4x^2 \ln \left( 1 + \frac{5}{2x^2 - 3x} \right).


Step 2: Approximation for ln(1+u)\ln(1 + u)

For small uu, ln(1+u)u\ln(1 + u) \approx u. Here, as xx \to \infty, 52x23x0\frac{5}{2x^2 - 3x} \to 0. So, we can approximate: ln(1+52x23x)52x23x.\ln \left( 1 + \frac{5}{2x^2 - 3x} \right) \approx \frac{5}{2x^2 - 3x}.

Substitute this back into the expression for lny\ln y: lny4x252x23x.\ln y \approx 4x^2 \cdot \frac{5}{2x^2 - 3x}.


Step 3: Simplify the expression

Factor x2x^2 from the denominator: 52x23x=5x2(23x).\frac{5}{2x^2 - 3x} = \frac{5}{x^2 \left( 2 - \frac{3}{x} \right)}.

As xx \to \infty, the term 3x0\frac{3}{x} \to 0, so: 52x23x52x2.\frac{5}{2x^2 - 3x} \to \frac{5}{2x^2}.

Thus: lny4x252x2=202=10.\ln y \approx 4x^2 \cdot \frac{5}{2x^2} = \frac{20}{2} = 10.


Step 4: Exponentiate to find yy

Since lny10\ln y \to 10, we have: ye10.y \to e^{10}.


Final Answer:

limx(1+52x23x)4x2=e10.\lim_{x \to \infty} \left( 1 + \frac{5}{2x^2 - 3x} \right)^{4x^2} = e^{10}.

Let me know if you'd like additional clarifications or further explanations!


Follow-Up Questions:

  1. How is the approximation ln(1+u)u\ln(1 + u) \approx u derived for small uu?
  2. Why is it valid to ignore higher-order terms in the denominator for large xx?
  3. What happens if the exponent 4x24x^2 were replaced with a smaller growth rate (e.g., xx)?
  4. Could this problem be solved using L’Hôpital’s Rule? Why or why not?
  5. How can you generalize this approach for limits of the form limx(1+ab(x))c(x)\lim_{x \to \infty} \left(1 + \frac{a}{b(x)}\right)^{c(x)}?

Tip:

When evaluating limits involving exponential forms, always check if the base approaches 11 and the exponent grows without bounds—this indicates the use of logarithms and approximations!

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Math Problem Analysis

Mathematical Concepts

Exponential Growth
Limits and Continuity
Logarithmic Approximation

Formulas

ln(1 + u) ≈ u for small u
lim (x -> infinity) (1 + a/b(x))^c(x) = e^(a * lim (x -> infinity) c(x)/b(x))

Theorems

Logarithmic Approximation Theorem

Suitable Grade Level

Undergraduate Level