Chapter 3: Problem 44
Find the limits. $$ \lim _{x \rightarrow 0^{+}}(-\ln x)^{x} $$
Short Answer
Expert verified
The limit is 1.
Step by step solution
01
Understand the Problem
We need to find the limit of the function \((-\ln x)^x\) as \(x\) approaches 0 from the positive side. This means we will consider values of \(x\) that are positive and very close to 0.
02
Simplify the Expression
Let's express \((-\ln x)^x\) as an exponential function. Define \(y = (-\ln x)^x\). Taking the natural logarithm of both sides, we have \(\ln y = x \ln(-\ln x)\).
03
Evaluate the Limit of the Logarithm
We evaluate \(\lim_{x \to 0^+} x \ln(-\ln x)\). As \(x \to 0^+\), \(-\ln x\) approaches infinity, thus \(\ln(-\ln x)\) approaches infinity. We have the form \(0 \cdot \infty\), which is indeterminate. We need L'Hôpital's Rule here.
04
Apply L'Hôpital's Rule
Rewrite \(x \ln(-\ln x)\) as \(\frac{\ln(-\ln x)}{1/x}\). Apply L'Hôpital's Rule by differentiating the numerator and denominator. The derivatives are \(\frac{d}{dx}[\ln(-\ln x)] = \frac{-1}{x \ln x}\) and \(\frac{d}{dx}[1/x] = -1/x^2\).
05
Simplify Derivative Result
Now apply L'Hôpital's Rule: \(\lim_{x \to 0^+} \frac{\ln(-\ln x)}{1/x} = \lim_{x \to 0^+} \frac{-1/(x \ln x)}{-1/x^2} = \lim_{x \to 0^+} \frac{x}{\ln x}\).
06
Find the Limit of Result
Notice \(\lim_{x \to 0^+} \frac{x}{\ln x} \) also tends to \(0\) because the exponential decrease of \(\ln x\) dominates any linear decrease in \(x\). So this limit is 0.
07
Conclude the Original Limit
Since \(\lim_{x \to 0^+} x \ln(-\ln x) = 0\), the original limit becomes \(e^0 = 1\). This means \(\lim_{x \to 0^+} (-\ln x)^x = 1\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
L'Hôpital's Rule
L'Hôpital's Rule is a powerful tool used in calculus to evaluate limits of indeterminate forms. When you encounter a limit that results in a form such as \(0/0\) or \(\infty/\infty\), this rule helps by allowing you to differentiate the numerator and the denominator separately.
Once differentiated, you can then re-evaluate the limit.Here's how it works in practice:
Once differentiated, you can then re-evaluate the limit.Here's how it works in practice:
- Start with an indeterminate form—this is when the limit of a function results in \(0/0\) or \(\infty/\infty\).
- Differentiate the top and bottom of the fraction separately.
- Re-evaluate the limit with these new expressions.
Exponential Functions
Exponential functions play a central role in so many areas of mathematics. Generally, these functions are in the form \(f(x) = a^x\), where \(a\) is a constant base, and \(x\) is the exponent.In the context of limits and calculus, exponential functions often manifest as part of complex expressions that need simplification. Here, initially, the expression \((-\ln x)^x\) was examined.
This expression was transformed into the form \(e^{x \ln(-\ln x)}\) by utilizing the properties of logarithms and exponentials:
This expression was transformed into the form \(e^{x \ln(-\ln x)}\) by utilizing the properties of logarithms and exponentials:
- The function \(y = (-\ln x)^x\) can be rewritten as \(\ln y = x \ln(-\ln x)\), simplifying our calculations.
- This step converts a potentially confusing exponential power into more manageable algebraic and logarithmic forms.
Indeterminate Forms
Indeterminate forms refer to expressions that, at first glance, do not have a clear limit. In calculus, these forms include \(0/0\), \(\infty/\infty\), \(0^0\), \(\infty - \infty\), among others.
These forms are called "indeterminate" because they don't imply a specific outcome given their superficial expression.In our problem, while assessing \(x \ln(-\ln x)\), we encountered the \(0 \cdot \infty\) form, an example of an indeterminate form:
These forms are called "indeterminate" because they don't imply a specific outcome given their superficial expression.In our problem, while assessing \(x \ln(-\ln x)\), we encountered the \(0 \cdot \infty\) form, an example of an indeterminate form:
- Indeterminate forms are not resolved by simple arithmetic or algebraic manipulation.
- Advanced mathematical techniques, like L'Hôpital's Rule, are usually applied to work through these forms.
- Recognizing an indeterminate form is pivotal since it tells you that additional methods need to be employed to find the limit.