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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 11.2.30

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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Identify the given power series: \(\sum_{k=0}^{\infty} k (x - 1)^k\). This is a power series centered at \(x = 1\).
To find the radius of convergence, use the Ratio Test. Consider the general term \(a_k = k (x - 1)^k\). Compute the limit \(L = \lim_{k \to \infty} \left| \frac{a_{k+1}}{a_k} \right|\).
Calculate the ratio inside the limit: \(\left| \frac{(k+1)(x-1)^{k+1}}{k (x-1)^k} \right| = \left| \frac{k+1}{k} \right| \cdot |x-1| = \left(1 + \frac{1}{k}\right) |x-1|\).
Take the limit as \(k \to \infty\): \(L = \lim_{k \to \infty} \left(1 + \frac{1}{k}\right) |x-1| = |x-1|\). The Ratio Test says the series converges if \(L < 1\), so the radius of convergence \(R\) satisfies \(|x-1| < 1\).
Determine the interval of convergence by checking the endpoints \(x = 1 - 1 = 0\) and \(x = 1 + 1 = 2\). Substitute these values into the series and test for convergence using appropriate tests (e.g., p-series, alternating series, or divergence test).

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Key Concepts

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

Power Series

A power series is an infinite sum of terms in the form a_k(x - c)^k, where a_k are coefficients and c is the center. Understanding the structure of power series is essential to analyze their convergence behavior around the center point.
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Intro to Power Series

Radius of Convergence

The radius of convergence is the distance from the center within which the power series converges absolutely. It can be found using tests like the Ratio Test or Root Test, and it defines the interval where the series behaves well.
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Radius of Convergence

Interval of Convergence

The interval of convergence includes all x-values for which the power series converges, typically centered at c and extending radius R in both directions. Endpoints must be checked separately to determine if the series converges or diverges there.
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Interval of Convergence
Related Practice
Textbook Question

Derivative trick Here is an alternative way to evaluate higher derivatives of a function f that may save time. Suppose you can find the Taylor series for f centered at the point a without evaluating derivatives (for example, from a known series). Then f⁽ᵏ⁾(a)=k! multiplied by the coefficient of (x−a)ᵏ. Use this idea to evaluate f⁽³⁾(0) and f⁽⁴⁾(0) for the following functions. Use known series and do not evaluate derivatives.


f(x) = ∫₀ˣ sin t² dt

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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

Power series for derivatives


a. Differentiate the Taylor series centered at 0 for the following functions.

b. Identify the function represented by the differentiated series.

c. Give the interval of convergence of the power series for the derivative.


f(x) = eˣ

Textbook Question

Differentiating and integrating power series Find the power series representation for g centered at 0 by differentiating or integrating the power series for f (perhaps more than once). Give the interval of convergence for the resulting series.


g(x) = x/(1 + x²)² using f(x) = 1/(1 + x²)

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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

Limits Evaluate the following limits using Taylor series.

lim ₓ→₄ (x² 16)/(ln (x 3)}

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