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Ch. 11 - Power Series
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243당신이 사용하는 게 아니라요?교과서 변경
11장, 문제 11.2.15

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


∑ₖ₌₀∞ (x/3)ᵏ

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1
Identify the given power series: \( \sum_{k=0}^{\infty} \left( \frac{x}{3} \right)^k \). This is a geometric series with the common ratio \( r = \frac{x}{3} \).
Recall that a geometric series \( \sum_{k=0}^{\infty} r^k \) converges if and only if \( |r| < 1 \). Apply this condition to the given series: \( \left| \frac{x}{3} \right| < 1 \).
Solve the inequality \( \left| \frac{x}{3} \right| < 1 \) to find the interval of convergence. Multiply both sides by 3 to get \( |x| < 3 \). This means the radius of convergence \( R = 3 \).
Express the interval of convergence as \( (-3, 3) \). Next, check the endpoints \( x = -3 \) and \( x = 3 \) by substituting them back into the series to determine if the series converges at these points.
At \( x = 3 \), the series becomes \( \sum_{k=0}^{\infty} 1^k \), which diverges. At \( x = -3 \), the series becomes \( \sum_{k=0}^{\infty} (-1)^k \), which also diverges. Therefore, the interval of convergence is \( (-3, 3) \) without including the endpoints.

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

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 power series involves recognizing how the variable x affects convergence depending on its distance from the center.
추천 영상:
05:58
Intro to Power Series

Radius of Convergence

The radius of convergence is the distance from the center within which a power series converges absolutely. It can be found using tests like the Ratio Test, and it defines the interval where the series behaves well.
추천 영상:
07:36
Radius of Convergence

Interval of Convergence

The interval of convergence is the set of x-values for which the power series converges. It includes all points within the radius of convergence and requires checking endpoints separately to determine if they are included.
추천 영상:
08:44
Interval of Convergence