Problem 21 - Calculus with Concepts in Calculus

Question

Find the given limit. $$ \lim _{x \rightarrow \infty} \frac{1}{\ln x} $$

Step-by-Step Solution

Answer

The limit is 0.

Step 1: Identify the limit expression

We need to find the limit of the function \( f(x) = \frac{1}{\ln x} \) as \( x \to \infty \).

Step 2: Analyze the behavior of \( \ln x \)

As \( x \to \infty \), the natural logarithm \( \ln x \to \infty \) because the logarithmic function increases without bound as its argument increases.

Step 3: Evaluate the reciprocal

Since \( \ln x \to \infty \), the reciprocal \( \frac{1}{\ln x} \) goes to \( 0 \) because the reciprocal of a function that tends to infinity approaches zero.

Step 4: Conclude the limit

Putting these observations together, we see that \( \lim_{x \to \infty} \frac{1}{\ln x} = 0 \).

Key Concepts

natural logarithm

The natural logarithm, denoted as \( \ln x \), is a special logarithm that uses the base \( e \), where \( e \) is an irrational and transcendental number approximately equal to 2.71828. It is particularly important in calculus and natural growth processes. Unlike other logarithmic bases, the natural logarithm correlates directly with exponential functions, which are prevalent in many areas of science and engineering.
The natural logarithm has specific behaviors:

• It is only defined for positive values of \( x \).
• As \( x \) approaches 0 from the positive side, \( \ln x \) tends to \( -\infty \).
• As \( x \) approaches infinity, \( \ln x \) tends to \( \infty \).

This leads to the fact that the natural logarithm grows, but does so slowly compared to linear or polynomial growth. This property is essential when evaluating limits involving \( \ln x \), as it frequently appears in problems dealing with large values of \( x \).

reciprocal

The reciprocal of a number \( y \) is simply \( \frac{1}{y} \). Applying this to functions like \( f(x) = \ln x \), the reciprocal becomes \( \frac{1}{\ln x} \). Understanding reciprocals is crucial in understanding behavior limits as they can significantly change the outcome.
Here are some important points regarding reciprocals:

• If a function approaches infinity, such as \( \ln x \) as \( x \) approaches infinity, its reciprocal approaches zero.
• If a function approaches zero, its reciprocal approaches infinity.
• The reciprocal flips the behavior of approaching zero and infinity.

In practical terms, when evaluating expressions like \( \lim_{x \to \infty} \frac{1}{\ln x} \), notice how the reciprocal transforms from potentially infinite growth of \( \ln x \) to the diminutive effect (approaching zero) of \( \frac{1}{\ln x} \). This understanding allows us to predict limits of functions that might be difficult to solve otherwise.

infinity behavior

Infinity behavior refers to how functions behave as their inputs grow very large (positively or negatively). In this context, understanding infinity behavior is key to analyzing limits such as \( \lim_{x \to \infty} f(x) \). Understanding whether a function approaches a finite number, infinity, or zero as \( x \) tends to infinity is a major aspect of calculus.
When examining bounds and growth:

• Functions like \( \ln x \) display unbounded growth as \( x \) approaches infinity, which means they will continue to grow without any upper limit.
• Infinity can relate to acceleration (e.g., exponential growth) or slower, steady growth (e.g., logarithmic).
• For a reciprocal function like \( \frac{1}{\ln x} \), understanding that it approaches zero due to the infinity behavior of \( \ln x \) is crucial.

Grasping infinity behavior helps in simplifying complex limit expressions and predicting their end behavior. This forms a fundamental skill in calculus where predicting these outcomes without computation can save time on tests and in real-world applications.