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Prove that $$\quad \lim _{x \rightarrow \infty} \frac{\ln x}{x^{p}}=0$$ for any number \(p>0 .\) This shows that the logarithmic function approaches infinity more slowly than any power of \(x .\)

Short Answer

Expert verified
The limit is 0, as the logarithm grows slower than any power of \(x\).

Step by step solution

01

Understanding the Problem

We need to show that as \(x\) approaches infinity, the ratio \(\frac{\ln x}{x^p}\) approaches 0 for any number \(p > 0\). This involves understanding how the natural logarithm \(\ln x\) behaves in comparison to any power function \(x^p\).
02

Using L'Hôpital's Rule

Since both the numerator \(\ln x\) and the denominator \(x^p\) approach infinity as \(x\) approaches infinity, we can use L'Hôpital's Rule which states that for functions \(f(x)\) and \(g(x)\) both approaching infinity, \(\lim_{x \to \infty} \frac{f(x)}{g(x)} = \lim_{x \to \infty} \frac{f'(x)}{g'(x)}\) if this limit exists.
03

Differentiate the Numerator and Denominator

The derivative of \(\ln x\) is \(\frac{1}{x}\), and the derivative of \(x^p\) is \(px^{p-1}\). So, apply L'Hôpital's Rule:\[\lim_{x \to \infty} \frac{\ln x}{x^p} = \lim_{x \to \infty} \frac{\frac{1}{x}}{px^{p-1}}\]
04

Simplifying the Expression

Simplify the expression from Step 3:\[\frac{\frac{1}{x}}{px^{p-1}} = \frac{1}{px^p}\]Now consider the limit as \(x\) approaches infinity:\[\lim_{x \to \infty} \frac{1}{px^p} = 0\]
05

Conclude the Proof

Since \(\lim_{x \to \infty} \frac{1}{px^p} = 0\), by L'Hôpital's Rule, it follows that \(\lim_{x \to \infty} \frac{\ln x}{x^p} = 0\). This means that the logarithm \(\ln x\) increases at a slower rate than any power of \(x\) as \(x\) approaches infinity.

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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 helpful tool in calculus for evaluating limits, especially when dealing with indeterminate forms like \(\frac{0}{0}\) or \(\frac{\infty}{\infty}\). It allows us to transform a complex problem into a more manageable one by differentiating the numerator and denominator separately.

  • To use L'Hôpital's Rule, first confirm that the initial limit results in an indeterminate statement.
  • Differentiate the numerator and the denominator separately.
  • Evaluate the new limit of the resulting function.
In our exercise, both \(\ln x\) and \(x^p\) tend towards infinity as \(x\) approaches infinity, resulting in the indeterminate form \(\frac{\infty}{\infty}\). Thus, L'Hôpital's Rule lets us differentiate \(\ln x\), which becomes \(\frac{1}{x}\), and \(x^p\), which becomes \(px^{p-1}\). The limit then reduces to \(\lim_{x \to \infty} \frac{\frac{1}{x}}{px^{p-1}}\), simplifying our problem effectively.
Natural Logarithm
The natural logarithm, denoted as \(\ln x\), is a logarithm with the base \(e\), where \(e\) is approximately equal to 2.718. It is a fundamental concept in mathematical analysis, often found in problems involving growth and decay.

  • \(\ln x\) represents the power to which \(e\) must be raised to obtain the value \(x\).
  • As \(x\) increases, \(\ln x\) also increases, but at a decreasing rate compared to power functions.
  • The derivative of \(\ln x\) is \(\frac{1}{x}\), which plays a key role in calculus exercises, especially when applying L'Hôpital's Rule.
In our exercise, \(\ln x\) is compared to \(x^p\). Despite both approaching infinity, the growth of \(\ln x\) is slower, especially when \(x\) becomes very large. This gradual increase signifies that over time, even the smallest positive power of \(x\) will exceed the rate at which \(\ln x\) grows.
Power Function Limits
Power functions are expressions of the form \(x^p\), where \(p\) is any real positive number. They are prominent in calculus due to their straightforward growth patterns and powerful implications in limit problems.

  • As \(x\) approaches infinity, power functions like \(x^p\), grow rapidly.
  • Even if \(p\) is very small and positive, \(x^p\) still increases faster than \(\ln x\).
  • This property is central to proving limits where logarithmic functions are involved.
In our particular exercise, comparing \(\ln x\) and \(x^p\), the result of applying L'Hôpital's Rule shows a stark contrast in their growth rates. While \(\ln x\) grows, any positive power function \(x^p\) outpaces it in the long run. Thus, \(\lim_{x \to \infty} \frac{\ln x}{x^p} = 0\) demonstrates the powerful limiting behavior of power functions, affirming that they grow significantly faster than logarithmic functions as \(x\) increases.

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