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Ch. 10 - Infinite Sequences and Series
Hass - Thomas' Calculus 15th Edition
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10장, 문제 10.7.28a

Intervals of Convergence
In Exercises 1–36, (a) find the series’ radius and interval of convergence.
∑ (from n = 0 to ∞) [ (−2)ⁿ (n + 1) (x − 1)ⁿ ]

검증된 단계별 안내
1
Identify the general term of the series: \(a_n = (-2)^n (n + 1) (x - 1)^n\).
Apply the Ratio Test to find the radius of convergence. Compute the limit \(L = \lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right|\).
Substitute \(a_{n+1}\) and \(a_n\) into the ratio: \(\left| \frac{(-2)^{n+1} (n+2) (x-1)^{n+1}}{(-2)^n (n+1) (x-1)^n} \right| = \left| -2 \cdot \frac{n+2}{n+1} \cdot (x-1) \right|\).
Simplify the expression inside the limit: \(L = \lim_{n \to \infty} 2 \cdot \frac{n+2}{n+1} \cdot |x-1| = 2 |x-1|\).
Set the limit \(L < 1\) for convergence, which gives \(2 |x-1| < 1\). Solve this inequality to find the radius of convergence and the interval centered at \(x=1\).

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주요 개념

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

Radius of Convergence

The radius of convergence is the distance from the center of a power series within which the series converges absolutely. It can be found using the Ratio Test or Root Test on the general term of the series. This radius defines an interval around the center where the series behaves well.
추천 영상:
07:36
Radius of Convergence

Interval of Convergence

The interval of convergence is the set of all x-values for which the power series converges. It is centered at the series’ expansion point and extends to the radius of convergence on both sides. Endpoints must be checked separately to determine if the series converges or diverges there.
추천 영상:
08:44
Interval of Convergence

Ratio Test for Series Convergence

The Ratio Test determines convergence by examining the limit of the absolute value of the ratio of consecutive terms. If this limit is less than one, the series converges absolutely; if greater than one, it diverges. This test is especially useful for power series to find the radius of convergence.
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