Writing math formula

Question

Currently I'm having to write something like:

I'm writing "q" in the denominator as |\vec{q}| using the "pipe"; is it the proper way to do it?

Answer 1

One way to do it is the following.

Code

\documentclass{article}

\usepackage{mathtools}
\DeclarePairedDelimiter{\abs}{\lvert}{\rvert}

\begin{document}
\[
\frac{\vec{q} \cdot \vec{r}_j}{\abs{\vec{q}\,} \times \abs{\vec{r}_j}}
\]
\end{document}

Output

Answer 2

As far as I'm into LaTeX this is correct. I would put your formular as

$=\frac{\vec{q}\cdot\vec{r}_j}{|\vec{q}|\times|\vec{r}_j|}$

So for getting the absolute value typing the pipe | is fine. But do not use the pipe for some expression like

{a \in A | a \notin B}

instead use \mid there.