Number only some equations in an equation array

Question

I have an equation array consisting of around 9 equations, however I wish to number only some of them. How can I do that?

Answer 1

You can use \notag and/or \nonumber from amsmath (or improvised mathtools package):

\documentclass{article}
\usepackage{mathtools,amssymb}


\begin{document}
\begin{align}
  x &= y \\\nonumber
  x &= 3 \\
  y &= 10 \\ \notag
 \therefore\ x &\neq y \\
   x &= y \\\nonumber
  x &= 3 \\
  y &= 10 \\ \notag
 \therefore\ x &\neq y \\
 ? &=?
\end{align}
\end{document}