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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 11.2.20

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


∑ₖ₌₀∞ (2x)ᵏ/k!

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Identify the given power series: \( \sum_{k=0}^{\infty} \frac{(2x)^k}{k!} \). This is a power series centered at 0 with general term \( a_k = \frac{(2x)^k}{k!} \).
To find the radius of convergence, use the Ratio Test. Consider the limit \( L = \lim_{k \to \infty} \left| \frac{a_{k+1}}{a_k} \right| \). Substitute \( a_k \) and \( a_{k+1} \): \[ L = \lim_{k \to \infty} \left| \frac{(2x)^{k+1} / (k+1)!}{(2x)^k / k!} \right| = \lim_{k \to \infty} \left| \frac{(2x)^{k+1}}{(k+1)!} \cdot \frac{k!}{(2x)^k} \right| = \lim_{k \to \infty} \left| \frac{2x}{k+1} \right| \]
Evaluate the limit \( L \). Since \( \lim_{k \to \infty} \frac{2|x|}{k+1} = 0 \), the Ratio Test tells us the series converges for all real \( x \).
Because the limit \( L = 0 < 1 \) for all \( x \), the radius of convergence \( R = \infty \). This means the power series converges for every real number \( x \).
Therefore, the interval of convergence is \( (-\infty, \infty) \). No endpoints need to be checked since the series converges everywhere.

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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 helps analyze functions represented as infinite polynomials and is essential for determining convergence properties.
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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 or Root Test, indicating the interval where the series represents a valid function.
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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 the series converges or diverges there.
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Interval of Convergence