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Ch. 10 - Infinite Sequences and Series
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077당신이 사용하는 게 아니라요?교과서 변경
10장, 문제 10.5.6

Using the Ratio Test
In Exercises 1–8, use the Ratio Test to determine whether each series converges absolutely or diverges.
∑(from n=2 to ∞) [(3ⁿ⁺²) / ln(n)]

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1
Identify the general term of the series as \(a_n = \frac{3^{n+2}}{\ln(n)}\) for \(n \geq 2\).
Write the Ratio Test limit expression: compute \(L = \lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right|\).
Substitute \(a_{n+1}\) and \(a_n\) into the ratio: \(\frac{a_{n+1}}{a_n} = \frac{\frac{3^{(n+1)+2}}{\ln(n+1)}}{\frac{3^{n+2}}{\ln(n)}} = \frac{3^{n+3}}{\ln(n+1)} \cdot \frac{\ln(n)}{3^{n+2}}\).
Simplify the ratio expression: \(\frac{a_{n+1}}{a_n} = 3 \cdot \frac{\ln(n)}{\ln(n+1)}\).
Evaluate the limit \(L = \lim_{n \to \infty} 3 \cdot \frac{\ln(n)}{\ln(n+1)}\) and use the Ratio Test criteria: if \(L < 1\), the series converges absolutely; if \(L > 1\), the series diverges; if \(L = 1\), the test is inconclusive.

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주요 개념

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

Ratio Test

The Ratio Test is a method to determine the convergence or divergence of an infinite series by examining the limit of the absolute value of the ratio of consecutive terms. If this limit is less than 1, the series converges absolutely; if greater than 1, it diverges; if equal to 1, the test is inconclusive.
추천 영상:
05:35
Ratio Test

Absolute Convergence

A series converges absolutely if the series of the absolute values of its terms converges. Absolute convergence guarantees convergence of the original series and is a stronger condition than conditional convergence, simplifying the analysis of series with positive and negative terms.
추천 영상:
07:51
Choosing a Convergence Test

Behavior of Logarithmic and Exponential Functions in Series

Understanding how logarithmic functions grow slowly compared to exponential functions is crucial when analyzing series terms. In this series, the numerator grows exponentially while the denominator grows logarithmically, affecting the limit in the Ratio Test and the overall convergence behavior.
추천 영상:
06:32
Derivatives of General Logarithmic Functions