# Difference between revisions of "009C Sample Midterm 1, Problem 1"

Does the following sequence converge or diverge?

If the sequence converges, also find the limit of the sequence.

$a_{n}={\frac {\ln n}{n}}$ Foundations:
L'Hôpital's Rule, Part 2

Let  $f$ and  $g$ be differentiable functions on the open interval  $(a,\infty )$ for some value  $a,$ where  $g'(x)\neq 0$ on  $(a,\infty )$ and  $\lim _{x\rightarrow \infty }{\frac {f(x)}{g(x)}}$ returns either  ${\frac {0}{0}}$ or  ${\frac {\infty }{\infty }}.$ Then,   $\lim _{x\rightarrow \infty }{\frac {f(x)}{g(x)}}=\lim _{x\rightarrow \infty }{\frac {f'(x)}{g'(x)}}.$ Solution:

Step 1:
First, notice that
$\lim _{n\rightarrow \infty }\ln n=\infty$ and
$\lim _{n\rightarrow \infty }n=\infty .$ Therefore, the limit has the form  ${\frac {\infty }{\infty }},$ which means that we can use L'Hopital's Rule to calculate this limit.
Step 2:
First, switch to the variable  $x$ so that we have functions and
can take derivatives. Thus, using L'Hopital's Rule, we have
${\begin{array}{rcl}\displaystyle {\lim _{n\rightarrow \infty }{\frac {\ln n}{n}}}&=&\displaystyle {\lim _{x\rightarrow \infty }{\frac {\ln x}{x}}}\\&&\\&{\overset {L'H}{=}}&\displaystyle {\lim _{x\rightarrow \infty }{\frac {{\big (}{\frac {1}{x}}{\big )}}{1}}}\\&&\\&=&\displaystyle {0.}\end{array}}$ The sequence converges. The limit of the sequence is  $0.$ 