Tuesday, May 10, 2016

sequences and series - How to prove that $displaystyle lim_{n to infty} sqrt[n]{n}=1$?




I have seen this fact thrown around a lot and never really stopped to prove it; plugging in a few values convinced me of its truth. But I would like to see the result proved. To be clear, this is not homework, just human curiosity; I'm looking for any nice proof. Thanks.


Answer



Rewrite as



$$
\sqrt[n]{n}=n^{1/n} = e^{\ln(n)/n}
$$
Now, you can write that




$$
\lim_{n\to\infty} \sqrt[n]{n} = \lim_{n\to\infty} e^{\ln(n)/n} = e^{\lim_{n\to\infty}\ln(n)/n}
$$
Looking at the exponent, you have (using L'Hopital's Rule)
$$
\lim_{n\to\infty}\frac{\ln(n)}n = \lim_{n\to\infty} \frac{1/n}{1}=\lim_{n\to\infty} \frac1n = 0
$$
Therefore, you have
$$
\lim_{n\to\infty} \sqrt[n]{n} = e^0 = 1

$$


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