I am trying to solve $$\lim \limits_{x \to 0}x\ln{x}$$ which according to WolframAlpha (and Wikipedia) equals $0$.
I managed to solve it by substituting such that $y = \dfrac{1}{x}$ and then using L'Hôpital's rule:
$$\begin{align}
\lim \limits_{x \to 0}x\ln{x}
& = \lim \limits_{y \to \infty}\frac{\ln{\frac{1}{y}}}{y} \\
& = \lim \limits_{y \to \infty}\frac{-\frac{1}{y^2}{\frac{1}{\frac{1}{y}}}}{1} \\
& = \lim \limits_{y \to \infty}\frac{-1}{y} \\
& = 0
\end{align}$$
but my question is when I try to solve it using L'Hôpital's rule without making the substitution, I get:
$$\begin{align}
& \lim \limits_{x \to 0}x\ln{x} \\
& = \lim \limits_{x \to 0}\frac{x}{x}+\ln{x} \\
& = \lim \limits_{x \to 0}1+\ln{x} \\
& = -\infty
\end{align}$$
So what went wrong here? Is it because I made $\frac{x}{x}=1$? If so how would I proceed from that point?
Or is this one of those cases where the caveat in L'Hôpital's rule that:
$$\lim \limits_{x \to c}\frac{f'(x)}{g'(x)}$$
has to exist is violated? Does equaling $-\infty$ count as not existing?
Best Answer
In short if the limit of $f$ and $g$ are both zero or both $\pm\infty$, and the limit $f'/g'$ exists, then the limit $f/g$ equals it.
What's wrong is the expression $x\ln x$ as you are implicitly defining $f$ and $g$ doesn't meet the hypothesis. However if we write it as
$$\frac{\ln x}{1/x}$$
we can use l'Hopital with $f(x) = \ln x$, $g(x) = 1/x$ as
Hence $\lim\limits_{x\to 0^+} x\ln x = 0$.