How can I label only one line of an equation array?

Question

Consider the following commands to create a labelled multi-line equation array:

\begin{eqnarray}
\label{eqn:1}
X&=&Y+Z\\
&\leq & U \times W.

\end{eqnarray}

It creates a two-line equations with two numbers. How can I associate only one equation number to this two-line equations? or better, how can I associated different equation numbers to each line? (of course, without breaking the equation array into several disjoint equations!)

Answer 1

amsmath provides a similar interface to eqnarray, but is better in terms of it's horizontal spacing/alignment:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align}
  X &= Y + Z \label{eqn:1} \\
    &\leq U \times W. \nonumber \\
    & = Y + Z \tag{a}
\end{align}
\end{document}

\nonumber removes the number for that line in the align environment. \tag can also be used to create a manual tag.

Answer 2

if only one equation number is wanted for a group, the split environment from amsmath is a reasonable candidate; it must be "wrapped" in an equation environment for the equation number to appear.

if all the equations in a group are to be numbered, then (as pointed out in other answers) the align environment is more suitable.

\begin{equation}
\begin{split}
 X &=Y+Z\\
   &\leq U \times W
\end{split}
 \label{eqn:1}
\end{equation}

\begin{align}
 X &=Y+Z \label{eqn:2}\\
   &\leq U \times W \label{eqn:3}
\end{align}

Answer 3

Use \notag:

\begin{eqnarray}
\label{eqn:1}
X&=&Y+Z\\
\notag
&\leq & U \times W.    
\end{eqnarray}

But, as @werner reminds us, it is not advised to use eqnarray, align does a better job:

\begin{align}
\label{eqn:1}
     X & = Y + Z \\
\notag & \leq  U \times W.    
\end{align}

This requires that you load the package amsmath to have access to \notag

Math environments in LaTeX are not happy with empty lines. If you want each line to have a number, just don't use \notag.

See How to get only one vertically centered equation number in align environment with two equations for how to get one equation number for multiple lines.