To add notation like Δx→0 underneath the limit (lim) notation (see below), you can use \lim\limits_{}.
\[ f_X(x)= \lim\limits_{\Delta x \rightarrow 0} \]
Generic Version (Base Version)
The following is the Latex code for the formula above.
f_X(x)= \lim\limits_{\Delta x \rightarrow 0}
For Markdown
If you want to show the formula above in R Markdown or Jupyter Markdown, you can use the following. The only difference is to add the dollar sign, namely $.
$f_X(x)= \lim\limits_{\Delta x \rightarrow 0}$
For WordPress
To show it in WordPress (like you see at the beginning of this post), you can use its short code and paste the following to show it.
Note that, the <script>...</script> part only need to be included on a post once, if you have multiple formulas to show.
<script type="text/javascript" async="" src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.4/MathJax.js?config=TeX-MML-AM_CHTML">
</script>
<p><span class="math display">\[ f_X(x)= \lim\limits_{\Delta x \rightarrow 0} \]</span></p>