# Find $\lim _{x \rightarrow 0} x \ln(x)$. Use l'Hopital's rule if it applies.

$$\lim _{x \rightarrow 0} x \ln(x)$$

I would say the limit will equal 1+ ln(x) but x cannot equal 0 at this point, so what shall i do ?

-
Could you say more about why you think the limit equals 1 + ln(x) (and what you even mean by that)? – mweiss Jul 8 '14 at 2:02
$1+\ln x$ is the derivative of $x\ln x$, but l'Hopital's rule for $\lim f(x)$ has nothing to do with the derivative of $f(x)$. – David Jul 8 '14 at 2:12
Remember that L'Hopital rule applies when you evaluate a limit and the result is an undetermined form ($0/0, \infty/\infty$) – AndrewGSM Jul 8 '14 at 4:16

## 4 Answers

You need to turn it into something that you can use l'Hopital on - that is, 0/0 or $\infty/\infty$.

For example, $$\frac{\ln(x)}{1/x}$$

-
So does l'Hopital's rule apply? – John Jul 8 '14 at 1:59
Well, what are the limits of the numerator and denominator of that expression as x approaches 0 from the right? – mweiss Jul 8 '14 at 2:01

Let $u=-\ln(x)$ which changes the problem to

$$- \lim_{u\to \infty} \frac{u}{e^u}.$$

Can you see it now?

-

$$\lim _{x \rightarrow 0} x \ln(x)=\lim _{x \rightarrow 0} \frac {\ln(x)}{\frac{1}{x}}=\lim _{x \rightarrow 0} \frac{\frac{1}{x}}{\frac{-1}{x^2}}=-\lim _{x \rightarrow 0}x=0$$

-

without L'Hopital

$\lim _{x \rightarrow 0} x \ln(x) = \lim_{x \rightarrow 0} \frac{\ln(x)}{1/x}$

then, $y = 1/x$ and when $x \rightarrow 0$, we know $y \rightarrow \infty$

then our limit is $\lim_{y \rightarrow \infty} \frac{\ln(1/y)}{y} = \lim_{y \rightarrow \infty} \frac{\ln(1)- \ln(y)}{y} = \lim_{y \rightarrow \infty} \frac{- \ln(y)}{y} = \lim_{y \rightarrow \infty} -\frac{1}{y} \ln(y)$

$\lim_{y \rightarrow \infty} -\ln(y ^ \frac{1}{y} )$

And We know $\lim_{y \rightarrow \infty} y ^ \frac{1}{y} = 1$ , then our limit
$\lim_{y \rightarrow \infty} -\ln(y ^ \frac{1}{y} ) = -\ln(1) = 0$

-