Math Problem Statement

Find the limit. Use L'Hospital's Rule where appropriate. If there is a more elementary method, consider using it. If L'Hospital's Rule doesn't apply, explain why. \[\lim_{{x \to 1}} \left( \frac{x}{x - 1} - \frac{1}{\ln x} \right)\]

Solution

To solve the given limit:

limx1(xx11lnx),\lim_{{x \to 1}} \left( \frac{x}{x - 1} - \frac{1}{\ln x} \right),

we need to determine if we can apply L'Hospital's Rule, which is typically used for indeterminate forms like 00\frac{0}{0} or \frac{\infty}{\infty}. Let's analyze each term individually as x1x \to 1.

Step 1: Analyzing xx1\frac{x}{x - 1}

As x1x \to 1, the denominator (x1)(x - 1) approaches zero, and the numerator xx approaches 1. This part alone does not yield a simple indeterminate form; however, xx1\frac{x}{x - 1} tends to infinity as xx approaches 1.

Step 2: Analyzing 1lnx\frac{1}{\ln x}

As x1x \to 1, lnx\ln x approaches zero from below (since ln(1)=0\ln(1) = 0), making 1lnx\frac{1}{\ln x} tend to -\infty.

Step 3: Combining Terms

Now, we have two terms: xx1and1lnx.\frac{x}{x - 1} \to \infty \quad \text{and} \quad \frac{1}{\ln x} \to -\infty. This creates an \infty - \infty indeterminate form, suggesting that L'Hospital's Rule might be useful.

Step 4: Rewriting and Applying L'Hospital's Rule

Rewrite the expression as a single fraction: limx1(xx11lnx)=limx1xlnx(x1)(x1)lnx.\lim_{{x \to 1}} \left( \frac{x}{x - 1} - \frac{1}{\ln x} \right) = \lim_{{x \to 1}} \frac{x \ln x - (x - 1)}{(x - 1) \ln x}.

Now, we can try to apply L'Hospital's Rule by differentiating the numerator and denominator with respect to xx. Differentiating:

  1. Numerator: The derivative of xlnx(x1)x \ln x - (x - 1) is lnx+11=lnx\ln x + 1 - 1 = \ln x.
  2. Denominator: The derivative of (x1)lnx(x - 1) \ln x is (\ln x + \frac{x - 1}{x} = \ln x + \frac{1 - 1}{1} = \ln

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Math Problem Analysis

Mathematical Concepts

Limits
Indeterminate Forms
L'Hospital's Rule

Formulas

L'Hospital's Rule: \(\lim_{{x \to a}} \frac{f(x)}{g(x)} = \lim_{{x \to a}} \frac{f'(x)}{g'(x)}\) if it results in \(\frac{0}{0}\) or \(\frac{\infty}{\infty}\)
Natural Logarithm: \(\ln(x)\)

Theorems

L'Hospital's Rule

Suitable Grade Level

College-level Calculus