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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 10.2.59

Which series in Exercises 53–76 converge, and which diverge? Give reasons for your answers. If a series converges, find its sum.
∑ (from n = 0 to ∞) e^(−2n)

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Identify the type of series given: \( \sum_{n=0}^{\infty} e^{-2n} \). Notice that this is a geometric series because each term can be written as \( (e^{-2})^n \).
Recall the formula for the sum of an infinite geometric series: if \( |r| < 1 \), then \( \sum_{n=0}^{\infty} ar^n = \frac{a}{1-r} \), where \( a \) is the first term and \( r \) is the common ratio.
Determine the first term \( a \) and the common ratio \( r \) for this series. Here, \( a = e^{-2 \cdot 0} = 1 \) and \( r = e^{-2} \).
Check the convergence condition by verifying if \( |r| < 1 \). Since \( e^{-2} \) is a positive number less than 1, the series converges.
Use the sum formula to express the sum of the series as \( \frac{1}{1 - e^{-2}} \). This represents the sum of the infinite series.

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Geometric Series

A geometric series is a series where each term is obtained by multiplying the previous term by a constant ratio r. It converges if the absolute value of r is less than 1, and its sum can be found using the formula S = a / (1 - r), where a is the first term.
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Convergence Criteria for Infinite Series

An infinite series converges if the sequence of its partial sums approaches a finite limit. For geometric series, this depends on the common ratio, while for other series, tests like the comparison or ratio test may be used to determine convergence or divergence.
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Exponential functions of the form e^(−kn) decrease rapidly as n increases when k > 0. When used as terms in a series, they often form geometric series with ratio e^(−k), which helps in analyzing convergence and calculating sums.
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