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How to show that $\lim_{n \rightarrow \infty} (\frac{n}{n+a})^n = \frac{1}{e^a}\,\,?$

If $a=1\,$ then $(\frac{n}{n+1})^n = (\frac{1}{\frac{n+1}{n}})^n = \frac{1}{(\frac{n+1}{n})^n} \rightarrow \frac{1}{e}$ as $ n \rightarrow \infty$

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    I don't get it. I know that $(\frac{n+1}{n})^n \rightarrow e$ and I know that $(\frac{n}{n+1})^n \rightarrow 1/e$2012-10-30

4 Answers 4

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Hint: Write your function as $\bigg(\frac{n}{n+a}\bigg)^n=\bigg(\frac{1}{1+\frac{a}{n}}\bigg)^n=\frac{1}{(1+\frac{a}{n})^n}$

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    ok. I thougth that you should use some kind of function like: if $1/f(n) = 1/e$ as n -> \infty, then $(1/f(n))^a = 1/e^a$ as n -> \infty,2012-10-30
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Rewrite your function as $\displaystyle e ^{-n\ln \left(1+\cfrac an\right)}$ and do a change of variable $t=\cfrac 1n$ where $t\rightarrow 0$ when $n\rightarrow +\infty$ then apply l'Hopital's.

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Note that - $\frac{n}{n+a}=\frac{n+a-a}{n+a}=1-\frac{a}{n+a}$ And therefore for any $a\in\mathbb{R}$: $\lim_{n\to\infty} \left(1-\frac{a}{n+a}\right)^n=\lim_{n\to\infty} \left(1-\frac{a}{n+a}\right)^{n+a}\cdot\left(1-\frac{a}{n+a}\right)^{-a}=$$=\lim_{n\to\infty} \left(1-\frac{a}{n+a}\right)^{n+a}\cdot\lim_{n\to\infty}\left(1-\frac{a}{n+a}\right)^{-a}=e^{-a}$

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    You're right - my mistake. I should have left it with $n+a$...2012-10-30
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for this case we can calculate Ln() of the function and then result is exp() of the answer.

$ \lim_{n \rightarrow \infty} (\frac{n}{n+a})^n $ so we have : $ \text{Ans}= \lim_{n \rightarrow \infty}\left[\ln \left(\frac{n}{n+a}\right)^n \right] =\lim_{n \rightarrow \infty} \left[ n. \ln \left(\frac{n}{n+a}\right) \right] = \infty . 0 $ $ \text{Ans}= \lim_{n \rightarrow \infty} \left[ \frac{\ln(\frac{n}{n+a})}{\frac{1}{n}} \right]=\frac{0}{0} \space \rightarrow \space \text{Ambiguity in Mathematics}$ we can use Hopital's rule,so we have:

$ \text{Ans}=\lim_{n \rightarrow \infty}\left[ \frac{\frac{a}{n(n+a)}}{\frac{-1}{n^2}} \right]=\lim_{n \rightarrow \infty} \left[ \frac{-an^2}{n(n+a)} \right]=\lim_{n \rightarrow \infty} \left[ \frac{-an}{(n+a)} \right] = -a $

$ \lim_{n \rightarrow \infty} (\frac{n}{n+a})^n = e^{\text{Ans}} =e^{-a}=\frac{1}{e^a}$