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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 11.4.16

Limits Evaluate the following limits using Taylor series.
lim ₓ→₄ (x² 16)/(ln (x 3)}

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First, rewrite the limit expression clearly: \(\lim_{x \to 4} \frac{x^2 - 16}{\ln(x - 3)}\).
Recognize that as \(x\) approaches 4, the numerator \(x^2 - 16\) approaches \(4^2 - 16 = 0\), and the denominator \(\ln(x - 3)\) approaches \(\ln(1) = 0\), so this is an indeterminate form \(\frac{0}{0}\) suitable for applying Taylor series expansions.
Expand the numerator \(x^2 - 16\) around \(x = 4\) using the Taylor series (or simply use the linear approximation): \(x^2 - 16 = (4)^2 - 16 + 2 \cdot 4 (x - 4) + \cdots = 0 + 8(x - 4) + \cdots\).
Expand the denominator \(\ln(x - 3)\) around \(x = 4\). Since \(x - 3\) approaches 1, use the expansion of \(\ln(1 + h)\) where \(h = x - 4\): \(\ln(x - 3) = \ln(1 + (x - 4)) = (x - 4) - \frac{(x - 4)^2}{2} + \cdots\).
Substitute these expansions back into the limit expression and simplify by canceling common factors, then evaluate the limit by taking \(x \to 4\) (or equivalently \(h \to 0\)).

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Key Concepts

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

Limits and Limit Evaluation

Limits describe the behavior of a function as the input approaches a particular value. Evaluating limits helps determine the function's value near points where direct substitution may be undefined or indeterminate, such as 0/0 or ∞/∞ forms.
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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 simplification of complex expressions to evaluate limits or analyze behavior.
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Handling Indeterminate Forms Using Series

When direct substitution in limits results in indeterminate forms like 0/0, expanding numerator and denominator into Taylor series helps identify leading terms. This approach simplifies the limit evaluation by canceling common factors and revealing the limit's value.
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Related Practice
Textbook Question

{Use of Tech} Newton's derivation of the sine and arcsine series Newton discovered the binomial series and then used it ingeniously to obtain many more results. Here is a case in point.

a. Referring to the figure, show that x = sin s or s = sin ⁻¹ x.

b. The area of a circular sector of radius r subtended by an angle θ is 1/2r²θ. Show that the area of the circular sector APE is s/2, which implies that

s = 2 ∫₀ˣ √(1 − t²) dt − x √(1 −x²)

c. Use the binomial series for f(x) = √(1 − x²) to obtain the first few terms of the Taylor series for s=sin ⁻¹ x.

d. Newton next inverted the series in part (c) to obtain the Taylor series for x=sin s. He did this by assuming sin s = ∑ aₖ sᵏ and solving x = sin(sin ⁻¹ x) for the coefficients aₖ. Find the first few terms of the Taylor series for sin s using this idea (a computer algebra system might be helpful as well).

Textbook Question

Radius and interval of convergence Determine the radius and interval of convergence of the following power series.

∑ₖ₌₀∞ k(x−1)ᵏ

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Textbook Question

Representing functions by power series Identify the functions represented by the following power series.

∑ₖ₌₀∞ 2ᵏ x²ᵏ⁺¹

Textbook Question

Working with binomial series Use properties of power series, substitution, and factoring to find the first four nonzero terms of the Maclaurin series for the following functions. Use the Maclaurin series


(1 + x)⁻² = 1 − 2x + 3x² − 4x³ + ⋯, for −1 < x < 1.


(1 + 4x)⁻²

Textbook Question

Approximating real numbers Use an appropriate Taylor series to find the first four nonzero terms of an infinite series that is equal to the following numbers.

√e

Textbook Question

Does the accuracy of an approximation given by a Taylor polynomial generally increase or decrease with the order of the approximation? Explain.

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