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Ch. 11 - Power Series
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Non è quello che usi tu?Cambia libro di testo
Capitolo 11, Problema 11.2.28

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


∑ₖ₌₂∞ ((x+3)ᵏ)/(k łn²k)

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1
Identify the given power series: \( \sum_{k=2}^{\infty} \frac{(x+3)^k}{k \ln^2(k)} \). Notice that the series is centered at \( x = -3 \) because the term is \( (x+3)^k \).
To find the radius of convergence, apply the Root Test or the Ratio Test. Here, the Root Test is convenient. Consider the general term \( a_k = \frac{(x+3)^k}{k \ln^2(k)} \). Compute \( \lim_{k \to \infty} \sqrt[k]{|a_k|} \).
Calculate \( \sqrt[k]{|a_k|} = \sqrt[k]{\frac{|x+3|^k}{k \ln^2(k)}} = |x+3| \cdot \sqrt[k]{\frac{1}{k \ln^2(k)}} \). As \( k \to \infty \), \( \sqrt[k]{k} \to 1 \) and \( \sqrt[k]{\ln^2(k)} \to 1 \), so the limit simplifies to \( |x+3| \).
The Root Test states that the series converges if \( \lim_{k \to \infty} \sqrt[k]{|a_k|} < 1 \), so the radius of convergence \( R \) satisfies \( |x+3| < 1 \). Thus, the radius of convergence is \( R = 1 \).
Next, determine the interval of convergence by checking the endpoints \( x = -3 - 1 = -4 \) and \( x = -3 + 1 = -2 \). Substitute these values into the original series and analyze convergence using appropriate tests (such as the Comparison Test or Integral Test) because the behavior at endpoints depends on the series without the \( (x+3)^k \) factor.

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Radius of Convergence

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

Interval of Convergence

The interval of convergence is the set of all x-values for which the power series converges. It includes the radius of convergence and requires checking endpoints separately, as convergence at these points is not guaranteed by the radius alone.
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08:44
Interval of Convergence

Ratio Test for 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; if greater than one, it diverges. It is especially useful for power series to find the radius of convergence.
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