In LaTeX, the command \bar{x} gives a short bar over a letter, which is okay for single characters like just x. But when you write something like \bar{AB}, the bar only covers the A, not the whole AB. That’s where the problem comes in.
To solve this, we use \widebar{AB}. This command puts a longer bar over the entire expression AB, which looks much neater.
How to use \widebar?
The \widebar is not available in LaTeX’s default packages, so we need to do a little extra work to use it.
For this, we can use packages like \usepackage{mathabx}, or sometimes \usepackage{extarrows}, or even with \usepackage{amsmath, amsfonts, amssymb}.
However, the easiest and most reliable way is to add the \mathabx package.
\documentclass{article}
\usepackage{mathabx}
\begin{document}
This is a normal bar: $\bar{AB}$
This is a long bar: $\widebar{AB}$
\end{document}
Conclusion
When you use the mathabx package, it may slightly change the appearance of some math symbols or fonts.
If that causes problems in your document, there are other ways to make a wide bar (like using \overline or special tricks).
