# Multiple align in equation with single equation number

I'm trying to get the following equation aligned two times per line and also I want to assign a single number to the equation. This is the code I have:

 \begin{alignat*}{2}
[M/H] &\cong\; 0.917 [Fe/H] &(1)\\
&\cong\; 0.977 [Fe/H] &(2)\\
&\cong\; [Fe/H] + log(0.638f_{\alpha}+0.362) \;\;\;&(3)
\end{alignat*}


This produces the properly aligned equations but no numbering. If I remove the asterisks I get one number for each line, which I don't want.

I've tried wrapping the above statement inside a  command, but it won't work. Ideas?

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Use alignedat* with the exact same syntax wrapped inside an equation environment. However the \; after \cong are wrong and "log" should be input as \log. –  egreg Jul 11 '12 at 15:55
The alignedat* command gives an undefined error. Are you sure that the right command? The \; are correct, I want those spaces in the equation. –  Gabriel Jul 11 '12 at 16:17
Sorry! It's alignedat without the * –  egreg Jul 11 '12 at 16:38

Use the alignedat environment:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{alignedat}{2} \textup{[M/H]} &\cong 0.917 \textup{[Fe/H]} &&(1)\\ &\cong 0.977 \textup{[Fe/H]} &&(2)\\ &\cong \textup{[Fe/H]} + \log(0.638f_{\alpha}+0.362) \quad&&(3) \end{alignedat}
\end{document}


The \; commands you're using are wrong, as they introduce an asymmetric space. Also the chemical symbols should be upright.

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This worked great, thanks! –  Gabriel Jul 11 '12 at 17:04