I know that the align environment labels all the lines, while the align* environment labels none of the lines. Is there a way to have the align environment label only some of the lines?
For example, suppose I have something like this:
\begin{align}
x+y
\\
x-y
\\
xy
\end{align}
This labels all three lines. Is there a way to only label, say, the second line (x-y)?