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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 10.8.59

11–86. Applying convergence tests Determine whether the following series converge. Justify your answers.
∑ (from k = 1 to ∞)k! / (kᵏ + 3)

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First, write down the general term of the series: \(a_k = \frac{k!}{k^k + 3}\).
To determine convergence, consider the behavior of \(a_k\) as \(k\) approaches infinity. Since \(k^k\) grows very rapidly, compare the growth rates of the numerator \(k!\) and the denominator \(k^k\).
Use the Ratio Test, which involves computing the limit \(L = \lim_{k \to \infty} \left| \frac{a_{k+1}}{a_k} \right|\). Substitute \(a_k\) and simplify the expression:
\[L = \lim_{k \to \infty} \frac{(k+1)!}{(k+1)^{k+1} + 3} \cdot \frac{k^k + 3}{k!}.\]
Simplify the factorial and powers, then evaluate the limit \(L\). If \(L < 1\), the series converges absolutely; if \(L > 1\), it diverges; if \(L = 1\), the test is inconclusive and another test should be applied.

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The Ratio Test evaluates the limit of the absolute value of the ratio of consecutive terms. If this limit is less than one, the series converges absolutely; if greater than one, it diverges. This test is especially useful for series involving factorials and exponential terms.
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