MHB  Use the Ratio Test for Convergence/Divergence

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The discussion focuses on applying the ratio test to determine the convergence or divergence of the series defined by the expression (1 - (1/k))^(3k) as k approaches infinity. The limit of the ratio of consecutive terms is calculated, yielding a result of 1, which is inconclusive for the ratio test. However, further analysis shows that the limit of the series terms approaches e^(-3), which is not zero. Therefore, the series is concluded to be divergent based on the term test. The application of both the ratio test and the term test illustrates the importance of multiple methods in assessing series convergence.
Fernando Revilla
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I quote a question from Yahoo! Answers

Use the ratio test to solve:? 1) k=1--> inf (1-(1/k))^(3k) 2)Is it convergent or divergent?

I have given a link to the topic there so the OP can see my response.
 
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We have

$\displaystyle\lim_{k\to +\infty}\frac{u_{k+1}}{u_k}=\lim_{k\to +\infty}\frac{\left(1-\frac{1}{k+1}\right)^{3k+3}}{\left(1-\frac{1}{k}\right)^{3k}}=\lim_{k\to +\infty}\left(1-\frac{1}{k+1}\right)^3\cdot\lim_{k\to +\infty}\frac{\left(1-\frac{1}{k+1}\right)^{3k}}{\left(1-\frac{1}{k}\right)^{3k}}$

$=1\cdot\displaystyle\lim_{k\to +\infty}\left(\frac{k^2}{k^2-1}\right)^{3k}=\ldots=1\cdot 1=1\text{ (inconclusive)}$

But $\displaystyle\lim_{k\to +\infty}u_k=\displaystyle\lim_{k\to +\infty}\left(1-\frac{1}{k}\right)^{3k}=\ldots=e^{-3}\ne 0$ so, (term test) the series is divergent.
 
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