equationsmathematics
How would I underline an equation with a curly brace and an explanation?, like I've pictured below?
I've tried to use the amsmath package, but the \underline directive does not produce the brace and creates ugly space between the A, B and =.
You could use the following, modified version of the code you've provided:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\left.
\begin{aligned}
x &= r\,\cos \varphi \quad\\ % use \quad as spacer between equation and right brace
y &= r\,\sin \varphi
\end{aligned}
\right\}
\end{equation}
\end{document}
By the way, both the align and split environments, which you employ in your code, let users set alignment points (e.g., the equal sign) with the & symbol. Using both environments simultaneously is redundant.
Note that I've substituted equation for align, and aligned for split, in the code above because you mention that you want the equations to be associated with a single equation number. (The align environment will create separate equation numbers for each equation it encounters unless you use the \notag command, but this seems more complicated than using the aligned environment inside an equation environment.)
The space is one of the lengths
\belowdisplayskip=12pt plus 3pt minus 9pt
\belowdisplayshortskip=7pt plus 3pt minus 4pt
so you can set those to whatever values you like using the above primitive syntax of \setlength
so the first tries to be 12pt but will shrink to 9pt or stretch to 21pt to help page breaking.
Best Answer
\underline is not \underbrace. Use