# Writing math formula

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?

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You should accept an answer if you find it satisfactory. –  Svend Tveskæg Mar 20 at 8:18

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

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A \vec without subscripts immediately followed by a bar clashes with it, so probably \abs{\vec{q}\,} is better. –  egreg Mar 13 at 11:27
Good point; I'll edit my answer. –  Svend Tveskæg Mar 13 at 11:28

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.

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