underline in align environment

A small problem this time: I try to underline my result in an align environment using the package ulem. The problem: I want to underline the whole result (e.g. x=1) not only 1. But because I like the = aligned, I have to write x&=1. If I want to underline it, the & is inside the \uuline{} and it gives an error.

Is there a solution for this? I found a "solution" online, but they put a line below outside and brought it in position via hspace and vspace, and that's a very dirty solution.

Here's an MWE, commented is the line which doesn't work.

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{ulem}

\begin{document}
Here's an example:
\begin{align*}
x+2&=3\\
x&=\uuline{1}
%\uuline{x&=1}
\end{align*}
\end{document}

• May I suggest it would look better boxed? The mathtools package (which loads amsmath) defines the \Aboxed{...} command, which accepts &. – Bernard Oct 8 '17 at 21:04
• Shouldn't "not online 1" be "not only 1"? I've omitted this edition, waiting for your decision. – Przemysław Scherwentke Oct 8 '17 at 21:14
• Yeah, it should be "not only 1", sorry. So in this case your solution given below is perfectly fine, thank you. – buecki Oct 9 '17 at 11:09

This underlines the second line of your pair, as required.

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{ulem}

\begin{document}
Here's an example:
\begin{align*}
x+2&=3\\
\makebox[0pt][l]{\uuline{\phantom{$x=1$}}}
x&=1
%x&=\uuline{1}
%\uuline{x&=1}
\end{align*}
\end{document}


If you need to use it many times, writing a macro should decrease the time needed.