On numerous occasions, I find myself trying to number the last line (and only
the last line!) of a long series of derivations in an align* environment. I
usually switch to the align environment and use \notag to turn off the
number on each line except the last.
There is a better solution, offered by egreg on StackOverflow:
By using equation and aligned, we get an environment similar to align*
that only numbers the last line.