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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 11.2.26

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


∑ₖ₌₁∞ (3x + 2)ᵏ/k

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Identify the given power series: \( \sum_{k=1}^{\infty} \frac{(3x + 2)^k}{k} \). This is a power series centered at the point where the expression inside the power is zero, so first find the center by solving \(3x + 2 = 0\).
Solve for \(x\) to find the center of the series: \(3x + 2 = 0 \implies x = -\frac{2}{3}\). This means the series is centered at \(x = -\frac{2}{3}\).
To find the radius of convergence, use the root test or ratio test. Here, the root test is convenient. Consider the general term \(a_k = \frac{(3x + 2)^k}{k}\). The root test involves calculating \( \lim_{k \to \infty} \sqrt[k]{|a_k|} \).
Calculate the limit: \( \lim_{k \to \infty} \sqrt[k]{\left| \frac{(3x + 2)^k}{k} \right|} = \lim_{k \to \infty} \frac{|3x + 2|}{\sqrt[k]{k}} = |3x + 2| \), since \( \sqrt[k]{k} \to 1 \) as \(k \to \infty\).
Set the limit less than 1 for convergence: \( |3x + 2| < 1 \). Solve this inequality for \(x\) to find the interval of convergence. Then, check the endpoints \( |3x + 2| = 1 \) separately by substituting back into the original series to determine if the series converges or diverges at those points.

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Key Concepts

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

Power Series and Convergence

A power series is an infinite sum of terms in the form a_k(x - c)^k, where c is the center. Understanding convergence means determining for which values of x the series sums to a finite value. This is essential to analyze the behavior of the series and find where it converges.
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Intro to Power Series

Radius of Convergence

The radius of convergence is the distance from the center c within which the power series converges absolutely. It can be found using tests like the Ratio Test or Root Test. Knowing the radius helps identify the interval on the x-axis 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 for convergence. This interval defines the domain where the series represents a valid function.
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Interval of Convergence
Related Practice
Textbook Question

L'Hôpital's Rule by Taylor series Suppose f and g have Taylor series about the point a.

a. If f(a) = g(a) = 0 and g′(a) ≠ 0, evaluate lim ₓ→ₐ f(x)/g(x) by expanding f and g in their Taylor series. Show that the result is consistent withl’Hôpital’s Rule.

b. If f(a) = g(a) =f′(a) = g′(a) = 0 and g′′(a) ≠ 0, evaluate lim ₓ→ₐ f(x)/g(x) by expanding f and g in their Taylor series. Show that the result is consistent with two applications of 1'Hôpital's Rule.

Textbook Question

{Use of Tech} Maximum error Use the remainder term to find a bound on the error in the following approximations on the given interval. Error bounds are not unique.


tan x ≈ x on [−π/6, π/6]

Textbook Question

Exponential function In Section 11.3, we show that the power series for the exponential function centered at 0 is


eˣ = ∑ₖ₌₀∞ (xᵏ)/k!, for −∞ < x < ∞


Use the methods of this section to find the power series centered at 0 for the following functions. Give the interval of convergence for the resulting series.


f(x) = e⁻³ˣ

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Textbook Question

Remainders Find the remainder Rₙ for the nth−order Taylor polynomial centered at a for the given functions. Express the result for a general value of n.


f(x) = sin x, a = π/2

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Textbook Question

{Use of Tech} Approximating powers Compute the coefficients for the Taylor series for the following functions about the given point a, and then use the first four terms of the series to approximate the given number.

f(x) =∛x with a=64; approximate ∛60.

Textbook Question

Inverse sine Given the power series

1/√(1 − x²) = 1 + (1/2)x² + (1 ⋅ 3)/(2 ⋅ 4) x⁴ + (1 ⋅ 3 ⋅ 5)/(2 ⋅ 4 ⋅ 6) x⁶ +⋯

for −1<x<1, find the power series for f(x) = sin ⁻¹ x centered at 0.

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