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Use l'Hôpital's rule to find $$ \lim _{x \rightarrow \infty}\left(1+\frac{c}{x}\right)^{x} $$ where \(c\) is a constant.

Short Answer

Expert verified
The limit is \(e^c\).

Step by step solution

01

Identify the form of the limit

First, recognize the type of indeterminate form as \(x \to \infty\), \(\left(1+\frac{c}{x}\right)^{x}\) takes the form \(1^{\infty}\). This suggests a logarithmic transformation can be beneficial.
02

Apply Logarithmic Transformation

Let \(y = \left(1+\frac{c}{x}\right)^{x}\). Take the natural logarithm of both sides to transform the expression: \(\ln y = x \cdot \ln\left(1+\frac{c}{x}\right)\). We will eventually exponentiate the result to find \(y\).
03

Set Up for L'Hôpital's Rule

Now consider the limit \(\lim_{x \to \infty} x \ln\left(1 + \frac{c}{x} \right)\). Rewrite it as a fraction that suggests L'Hôpital's Rule: \(\lim_{x \to \infty} \frac{\ln\left(1+\frac{c}{x}\right)}{1/x}\). This is now a \(\frac{0}{0}\) form suitable for applying L'Hôpital's Rule.
04

Apply L'Hôpital's Rule

Differentiate the numerator and the denominator separately: - The derivative of \(\ln\left(1 + \frac{c}{x}\right)\) with respect to \(x\) is \(-\frac{c}{x^2 + cx}\).- The derivative of \(1/x\) with respect to \(x\) is \(-1/x^2\).Applying L'Hôpital's Rule gives: \[ \lim_{x \to \infty} \frac{-\frac{c}{x^2 + cx}}{-\frac{1}{x^2}} = \lim_{x \to \infty} \frac{c x^2}{x^2 + cx} \].
05

Evaluate the Simplified Limit

Simplify the expression from Step 4:\[ \lim_{x \to \infty} \frac{c x^2}{x^2 + cx} = \lim_{x \to \infty} \frac{c}{1 + \frac{c}{x}} = c \] by dividing numerator and denominator by \(x^2\).
06

Solve the Original Expression

Recall that \(\ln y = c\) from Steps 2 to 5, so exponentiate to find \(y\):\[ y = e^{\ln y} = e^c \]. Therefore, the original limit is \(\lim_{x \to \infty} \left(1+\frac{c}{x}\right)^x = e^c\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Indeterminate Forms
In calculus, an indeterminate form is a mathematical expression that, due to its composition, does not lead directly to a well-defined limit. The most commonly encountered indeterminate forms include expressions like \( \frac{0}{0} \), \( \frac{\infty}{\infty} \), \( 0 \cdot \infty \), \( \infty - \infty \), as well as more complex forms like \( 1^{\infty} \), \( 0^{0} \), and \( \infty^{0} \). These forms are particularly interesting because they do not straightforwardly resolve to a specific value, necessitating additional mathematical techniques to evaluate them correctly.
For example, in our original exercise, we encounter the indeterminate form \( 1^\infty \). Although \( 1 \) raised to any finite power is \( 1 \), the situation becomes unclear when it's raised to an infinite power, thus making it indeterminate. To deal with such forms, mathematicians have developed methods like L'Hôpital's Rule and logarithmic transformations, which can help simplify and solve these intricate expressions.
Logarithmic Transformation
Logarithmic transformation is a powerful mathematical tool used to simplify expressions and resolve indeterminate forms. By taking the natural logarithm of a given expression, you can often transform a complex problem into a more manageable one. This technique is particularly useful when dealing with exponential expressions or when you encounter forms like \( 1^\infty \).
In the context of our problem, after recognizing this indeterminate form, we define \( y = \left(1 + \frac{c}{x}\right)^x \) and take the natural logarithm of both sides, yielding \( \ln y = x \ln\left(1 + \frac{c}{x}\right) \). This new expression allows us to convert the problem into a format that can be differentiated and evaluated using L'Hôpital's Rule.
The logarithmic transformation not only facilitates the application of calculus techniques but also simplifies exponential complexity, paving the way to find the desired limits.
Limits in Calculus
Limits are a fundamental concept in calculus, providing the foundation for understanding continuity, derivatives, and integrals. They describe the behavior of a function as its input approaches a particular value. Finding the limit often involves assessing what value a function approaches as the input becomes infinitely large or small.
In our example, we're dealing with a limit as \( x \rightarrow \infty \), implying we're interested in the behavior of \( \left(1 + \frac{c}{x}\right)^x \) as \( x \) grows without bound. Techniques like the logarithmic transformation and L'Hôpital's Rule allow us to evaluate this limit effectively. With large \( x \), \( \frac{c}{x} \to 0 \), turning the expression into \( (1+0)^x \), yet due to indeterminacy, deeper analysis is required. Using these methods helps us rigorously determine the limit as \( e^c \).
Understanding limits is crucial in calculus, providing insight into function behavior at boundaries and assisting in navigating through complex mathematical expressions.
Exponential Functions
Exponential functions involve expressions where a constant base is raised to a variable exponent, written generally as \( a^x \). These functions are vital in numerous fields, ranging from mathematics and physics to finance and biology. They describe phenomena involving growth or decay processes where the rate depends on the current value.
In our exercise, we initially encountered a form that eventually led to an exponential expression after resolving the indeterminate form. Through the process, we transformed \( \left(1 + \frac{c}{x}\right)^x \) and demonstrated that this expression approaches \( e^c \) as \( x \rightarrow \infty \), where \( e \) (approximately 2.718) is the base of natural logarithms.
Exponential functions are unique due to their growth rates and properties; for example, the derivative of \( e^x \) is \( e^x \), showcasing a distinct self-similarity property. These functions are fascinating because they model natural processes and provide a bridge between algebraic expressions and more complex calculus concepts.

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