Chapter 4: Problem 71
Determine whether the limit exists, and where possible evaluate it. \(\lim x^{a} \ln x,\) where \(a\) is \(x \rightarrow 0+\) a positive constant.
Short Answer
Expert verified
The limit \( \lim\limits_{x \to 0^+} x^a \ln x \) exists and equals 0.
Step by step solution
01
Rewrite the Limit Expression
The limit given is \( \lim\limits_{x \to 0^+} x^a \ln x \). Since \( a \) is a positive constant, the expression involves a power function and a logarithm.
02
Analyze the Behavior of \(x^a\)
As \( x \to 0^+ \), \( x^a \to 0 \) because \( a \) is a positive constant. This implies that \( x^a \) is a small positive number approaching zero.
03
Analyze the Behavior of \(\ln x\)
As \( x \to 0^+ \), \( \ln x \to -\infty \). This means \( \ln x \) becomes very large in the negative direction.
04
Combine and Assess Exponentially Small and Logarithmic Terms
The product \( x^a \ln x \) is a combination of an exponentially small term \( x^a \) and an unbounded negative term \( \ln x \). We need to determine the dominance of their behaviors as \( x \to 0^+ \).
05
Apply L'Hôpital's Rule
The form \( 0 \times -\infty \) can be transformed into \( \frac{0}{0} \) or \( \frac{-\infty}{\infty} \) using L'Hôpital's Rule. Rewriting \( x^a \ln x \) as \( \frac{\ln x}{1/x^a} \), we can apply L'Hôpital's Rule since both numerator and denominator approach infinity:\[ \lim\limits_{x \to 0^+} \frac{\ln x}{1/x^a} = \lim\limits_{x \to 0^+} \frac{1/x}{-a x^{-a-1}} \].
06
Simplify the Derivative
Calculate the derivatives:- Derivative of \( \ln x \) is \( \frac{1}{x} \).- Derivative of \( 1/x^{-a} \) is \( -a x^{-a-1} \).Thus, substitution gives us:\[ -a x^{a} \].
07
Evaluate the Limit
Now compute the limit:\[ \lim\limits_{x \to 0^+} -a x^{a} = 0 \]because as \( x \to 0^+ \), \( x^a \to 0 \) and thus the entire expression tends toward zero.
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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 handy tool in calculus that helps us analyze limits, especially those yielding indeterminate forms. An indeterminate form frequently encountered is the product of 0 and infinity, written as \(0 \times -\infty\). Though it seems unclear because one part is vanishing (0) and the other part is unbounded (\(-\infty\)), L'Hôpital's Rule provides a way out.
By reformulating the problem, we often switch the product into a quotient to obtain clearer conditions such as the forms \(\frac{0}{0}\) or \(\frac{\pm \infty}{\pm \infty}\). Once in a suitable form, take the derivative of both the numerator and denominator.
The rule states that:
By reformulating the problem, we often switch the product into a quotient to obtain clearer conditions such as the forms \(\frac{0}{0}\) or \(\frac{\pm \infty}{\pm \infty}\). Once in a suitable form, take the derivative of both the numerator and denominator.
The rule states that:
- If \(\lim\limits_{x \to c} \frac{f(x)}{g(x)}\) yields \(\frac{0}{0}\) or \(\frac{\pm \infty}{\pm \infty}\), then it can be evaluated as \(\lim\limits_{x \to c} \frac{f'(x)}{g'(x)}\), provided the result isn’t another indeterminate form.
logarithmic function
The logarithmic function is quite unique due to its behavior near certain critical points and its ubiquitous application across calculus. A natural logarithmic function, denoted generally as \(\ln x\), involves the inverse of an exponential function. As \(x\) approaches values close to zero from the positive side (denoted \(x \to 0^+\)), something interesting occurs: the logarithm approaches negative infinity (\(-\infty\)). This is important for evaluating calculus limits concerning such values of \(x\).
For a practical perspective:
For a practical perspective:
- As \(x\) decreases towards zero, \(\ln x\) becomes more negative, heading straight into negative territory.
- It is also continuous for all positive \(x\), meaning there are no breaks or jumps in its graph; it smoothly tends toward negative infinity as described above.
power functions
Power functions are common in all areas of mathematics, with a general form given by \(x^a\), where \(a\) represents a constant. These functions express relationships where variables are raised to a power.
When analyzing the behavior of power functions, especially for limits, examining how they behave around specific points on a number line is crucial.
Some core properties to keep in mind:
When analyzing the behavior of power functions, especially for limits, examining how they behave around specific points on a number line is crucial.
Some core properties to keep in mind:
- As \(x\) approaches zero from the positive side, and if \(a\) is a positive constant (like in our example), then \(x^a\) itself approaches zero. This behavior is because raising a small number to a power tends to shrink it even further.
- Power functions are continuous for all real numbers only if the base \(x\) is positive; this ensures the outcome is well-defined without any jumps or undefined areas.
- They are much "smoother" than logarithmic functions near zero, as they never head towards negative infinity on their own, only towards zero.