Chapter 10: Problem 26
Find the limit, if it exists. $$ \lim _{x \rightarrow 0} \frac{2 e^{x}-3 x-e^{-x}}{x^{2}} $$
Short Answer
Expert verified
The limit is \(\frac{3}{2}\).
Step by step solution
01
Recognize the Indeterminate Form
First, substitute \(x = 0\) into the function to check if it results in an indeterminate form:\[ \frac{2e^0 - 3(0) - e^0}{0^2} = \frac{2 - 1}{0} = \frac{1}{0}. \] The function appears to result in a form that suggests further analysis is needed because the denominator tends to zero.
02
Apply L'Hôpital's Rule
Since direct substitution gives us an indeterminate form \(\frac{0}{0}\), which suggests indeterminate behavior near \(x=0\), apply L'Hôpital's Rule:\[ \lim_{x \to 0} \frac{2e^x - 3x - e^{-x}}{x^2} = \lim_{x \to 0} \frac{\frac{d}{dx}(2e^x - 3x - e^{-x})}{\frac{d}{dx}(x^2)}.\] Differentiate the numerator and the denominator.
03
Differentiate the Numerator and Denominator
Differentiate the numerator:\[ \frac{d}{dx}(2e^x - 3x - e^{-x}) = 2e^x - 3 + e^{-x}. \]Differentiate the denominator:\[ \frac{d}{dx}(x^2) = 2x. \]
04
Evaluate the Limit After Differentiation
Use L'Hôpital's Rule to evaluate the limit after differentiation:\[ \lim_{x \to 0} \frac{2e^x - 3 + e^{-x}}{2x}. \] Substituting \(x = 0\) now gives:\[ \frac{2 - 3 + 1}{2 \times 0} = \frac{0}{0}, \] meaning we need to apply L'Hôpital's Rule again.
05
Apply L'Hôpital's Rule Again
Repeat the differentiation process since the result is still indeterminate:\[ \lim_{x \to 0} \frac{\frac{d}{dx}(2e^x - 3 + e^{-x})}{\frac{d}{dx}(2x)}. \] Differentiate the numerator and denominator again.
06
Differentiate Again
Perform the second differentiation on the numerator:\[ \frac{d}{dx}(2e^x - 3 + e^{-x}) = 2e^x + e^{-x}. \]Differentiate the denominator:\[ \frac{d}{dx}(2x) = 2. \]
07
Evaluate the Final Limit
Compute the limit with the new derivatives:\[ \lim_{x \to 0} \frac{2e^x + e^{-x}}{2}. \] Substitute \(x = 0\) into the expression:\[ \frac{2 \cdot 1 + 1}{2} = \frac{3}{2}. \] Thus, the limit exists and equals \(\frac{3}{2}.\)
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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 valuable tool in calculus for evaluating limits that produce indeterminate forms like \(\frac{0}{0}\) or \(\frac{\infty}{\infty}\). The rule applies when both the numerator and the denominator approach zero or infinity at a specific point. By differentiating the numerator and the denominator separately and then taking the limit of this new expression, you can often find a precise limit.
Practical Use
When faced with an indeterminate limit, follow these steps:- Confirm an indeterminate form exists by direct substitution.
- Apply L'Hôpital's Rule: Differentiate the numerator and denominator.
- Re-evaluate the limit after differentiation.
- Repeat if necessary, until a determinate value is achieved.
Indeterminate forms
Indeterminate forms arise in calculus when an expression does not immediately lead to a specific value with a conventional substitution. Common indeterminate forms include:
- \(\frac{0}{0}\)
- \(\frac{\infty}{\infty}\)
- \(0 \cdot \infty\)
- \(\infty - \infty\)
- \(0^0\)
- \(1^\infty\)
- \(\infty^0\)
Identifying Indeterminate Forms
In the original exercise, substituting \(x = 0\) into the function yielded \(\frac{0}{0}\). This indicated the limit needed more analysis, showcasing the usefulness of identifying indeterminate forms as a preliminary step in solving limits.Differentiation
Differentiation is a core concept in calculus used to find the rate at which a function is changing at any given point. When solving limits using L'Hôpital's Rule, differentiation allows us to rewrite the problematic limit in a simpler form.
Basic Steps of Differentiation
- Apply basic rules of differentiation: power, product, and chain rules.
- Differentiate each term in both the numerator and the denominator separately.
- Simplify the expression whenever possible to make further analysis easier.
Exponential functions
Exponential functions represent quantities that grow or decay at a rate proportional to their current value. The basic form is \(f(x) = e^x\), where \(e\) is Euler's number, approximately 2.71828. These functions have unique properties, particularly in differentiation.
Characteristics
- Exponential growth occurs when the base \(e\) is raised to positive exponents, resulting in a steep increase.
- Exponential decay happens with negative exponents, leading to a rapid decrease towards zero.
- The derivative of \(e^x\) is \(e^x\), preserving its exponential nature.