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Ch. 11 - Power Series
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Not the one you use?Change textbook
Chapter 11, Problem 11.4.14

Limits Evaluate the following limits using Taylor series.
lim ₓ→∞ x sin(1/x)

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1
Recognize that the limit involves the expression \(x \sin\left(\frac{1}{x}\right)\) as \(x\) approaches infinity, which suggests using the Taylor series expansion of \(\sin z\) around \(z=0\) where \(z = \frac{1}{x}\).
Recall the Taylor series expansion for \(\sin z\) around \(z=0\): \(\sin z = z - \frac{z^3}{3!} + \frac{z^5}{5!} - \cdots\)
Substitute \(z = \frac{1}{x}\) into the series: \(\sin\left(\frac{1}{x}\right) = \frac{1}{x} - \frac{1}{6x^3} + \frac{1}{120x^5} - \cdots\)
Multiply the entire series by \(x\): \(x \sin\left(\frac{1}{x}\right) = x \left( \frac{1}{x} - \frac{1}{6x^3} + \frac{1}{120x^5} - \cdots \right) = 1 - \frac{1}{6x^2} + \frac{1}{120x^4} - \cdots\)
Evaluate the limit as \(x \to \infty\) by observing that all terms with \(x\) in the denominator approach zero, so the limit is the constant term remaining in the expression.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Limits at Infinity

Limits at infinity describe the behavior of a function as the input grows without bound. Understanding how functions behave as x approaches infinity helps determine if the function approaches a finite value, infinity, or does not exist.
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Taylor Series Expansion

A Taylor series represents a function as an infinite sum of terms calculated from its derivatives at a single point. It approximates functions near that point, allowing complex expressions like sin(1/x) to be expanded into simpler polynomial terms for limit evaluation.
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Asymptotic Behavior of Functions

Asymptotic behavior studies how functions behave near specific points or at infinity. By analyzing dominant terms in expansions, one can simplify expressions like x sin(1/x) to find limits, focusing on leading terms that dictate the function's growth or decay.
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