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Evaluating a limit of infinity/infinity form using L'Hopital's Rule

Evaluate the limit:

lim⁡x→∞x+ln⁡(x)x−ln⁡(x)\lim_{x \to \infty} \frac{x + \ln(x)}{x - \ln(x)}

State the value of the limit.

Hint

First confirm that direct substitution gives the indeterminate form ∞/∞\infty/\infty. Then differentiate the numerator and denominator separately (do not use the quotient rule) and simplify before taking the limit again.

Step-by-step solution

As x→∞x \to \infty, both the numerator x+ln⁡(x)x + \ln(x) and denominator x−ln⁡(x)x - \ln(x) tend to ∞\infty, so the limit has the indeterminate form ∞/∞\infty/\infty. This confirms L'Hopital's Rule can be applied.

Differentiating numerator and denominator separately: the derivative of x+ln⁡(x)x + \ln(x) is 1+1x1 + \frac{1}{x}, and the derivative of x−ln⁡(x)x - \ln(x) is 1−1x1 - \frac{1}{x}. This gives the new limit lim⁡x→∞1+1/x1−1/x\lim_{x\to\infty} \frac{1 + 1/x}{1 - 1/x}.

As x→∞x \to \infty, 1/x→01/x \to 0, so the expression approaches 1+01−0=1\frac{1+0}{1-0} = 1. Therefore the original limit equals 11.

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