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Ch. 10 - Sequences and Infinite Series
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Non è quello che usi tu?Cambia libro di testo
Capitolo 10, Problema 10.1.63

61–66. Sequences of partial sums For the following infinite series, find the first four terms of the sequence of partial sums. Then make a conjecture about the value of the infinite series or state that the series diverges.


4 + 0.9 + 0.09 + 0.009 + ⋯  

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1
Identify the given infinite series: 4 + 0.9 + 0.09 + 0.009 + \(\cdots\). Notice that each term after the first is obtained by multiplying the previous term by 0.1, so this is a geometric series.
Write the first four terms explicitly: \(a_1 = 4\), \(a_2 = 0.9\), \(a_3 = 0.09\), and \(a_4 = 0.009\).
Calculate the first four partial sums \(S_n\), where \(S_n = a_1 + a_2 + \cdots + a_n\). So, \(S_1 = 4\), \(S_2 = 4 + 0.9\), \(S_3 = 4 + 0.9 + 0.09\), and \(S_4 = 4 + 0.9 + 0.09 + 0.009\).
Recognize that since this is a geometric series with first term \(a = 4\) and common ratio \(r = 0.1\), the sum of the first \(n\) terms can be expressed as \(S_n = a \frac{1 - r^n}{1 - r}\).
To conjecture the value of the infinite series, consider the limit of \(S_n\) as \(n \to \infty\). Since \(|r| < 1\), the infinite sum converges to \(S = \frac{a}{1 - r}\).

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