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Ch. 10 - Infinite Sequences and Series
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077Non è quello che usi tu?Cambia libro di testo
Capitolo 10, Problema 10.7.6

Intervals of Convergence
In Exercises 1–36, (a) find the series’ radius and interval of convergence. For what values of x does the series converge (b) absolutely, (c) conditionally?
∑ (from n = 0 to ∞) (2x)ⁿ

Guida verificata passo dopo passo
1
Identify the given series: \( \sum_{n=0}^{\infty} (2x)^n \). This is a geometric series with common ratio \( r = 2x \).
Recall that a geometric series \( \sum r^n \) converges if and only if \( |r| < 1 \). So, set up the inequality \( |2x| < 1 \) to find the interval of convergence.
Solve the inequality \( |2x| < 1 \) to get \( |x| < \frac{1}{2} \). This gives the radius of convergence \( R = \frac{1}{2} \) and the open interval \( (-\frac{1}{2}, \frac{1}{2}) \).
Check the endpoints \( x = -\frac{1}{2} \) and \( x = \frac{1}{2} \) by substituting into the series to determine if the series converges at these points. Since the series becomes \( \sum (-1)^n \) or \( \sum 1^n \), analyze their convergence behavior.
Determine absolute convergence by checking if the series \( \sum |(2x)^n| = \sum (2|x|)^n \) converges, which happens when \( |x| < \frac{1}{2} \). For conditional convergence, check if the series converges at endpoints where absolute convergence fails.

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Radius and Interval of Convergence

The radius of convergence is the distance from the center of a power series within which the series converges. The interval of convergence includes all x-values for which the series converges, possibly including endpoints. Finding these involves applying tests like the Ratio Test to determine where the series converges absolutely or conditionally.
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Radius of Convergence

Absolute Convergence

A series converges absolutely if the series of absolute values converges. This means ∑|a_n| converges, ensuring the original series converges regardless of term signs. Absolute convergence guarantees stronger convergence properties and often simplifies analysis of power series.
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Choosing a Convergence Test

Conditional Convergence

Conditional convergence occurs when a series converges, but not absolutely; that is, the series ∑a_n converges while ∑|a_n| diverges. This typically happens at the endpoints of the interval of convergence and requires careful testing, such as the Alternating Series Test, to determine convergence behavior.
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Choosing a Convergence Test