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I've looked around at various sites and tried switching between array/eqnarray but don't seem to be able to get the effect I would like. I want to simply give an array I have one single equation number. I would also like to know just to make sure, how to give each equation a number within an array.

The equation I have is the following:

$$
\begin{array}{lr}
N_{x,sk} = k_{sk}\left(\frac{t_{sk}}{b_{sk}}\right)^{2}\bar{Et} \;\;
N_{x,st} = k_{st}\left(\frac{t_{st}}{b_{st}}\right)^{2}\bar{Et}
\end{array}
$$

Could someone please help provide examples where I would have these equations showing one equation number and then also an example with two separate ones please?

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migrated from stackoverflow.com Nov 19 '12 at 15:24

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2 Answers

up vote 1 down vote accepted

Using the eqnarray environment instead of array, you can add \nonumber to any lines you don't want numbered. If you don't do this, every line will be numbered.

\begin{eqnarray}
N_{x,sk} &=& k_{sk}\left(\frac{t_{sk}}{b_{sk}}\right)^{2}\bar{Et} \nonumber \\
N_{x,st} &=& k_{st}\left(\frac{t_{st}}{b_{st}}\right)^{2}\bar{Et}
\end{eqnarray}

(Just remove the \nonumber from the above to get the second example.)

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Thank you, it's perfect! –  user18056 Nov 18 '12 at 19:00
    
Agreed. In the answer, I use eqnarray as the "vanilla" built-in multi-line equation alignment environment, but align, IEEEeqnarray, etc. are much better choices in general. –  ezod Nov 18 '12 at 21:06
1  
Please consider reading Peter Grill's answer, as use of eqnarray is not recommended. –  user2473 Nov 20 '12 at 2:34
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I would recommend you use the equation environment if you want a single equation for an entire array.

However, since this construct does not really seem to need an array, I would recommend using the align environment from the amsmath package, with which you can use \nonumber to selectively disable an equation number.

enter image description here

Notes:

  • Use of eqnarray is not recommended as per \eqnarray vs \align.

  • Also you should not use double dollar signs for display math as per Why is \[ … \] preferable to $$?

  • The math spacing in you array example was not correct. This has been corrected in the example below.

References:

Code:

\documentclass{article}
\usepackage{amsmath}

\begin{document}\noindent
Using \verb|equation| with \verb|array|:
\begin{equation}
\begin{array}{r@{}l}
    N_{x,sk} &{}= k_{sk}\left(\frac{t_{sk}}{b_{sk}}\right)^{2}\bar{Et}\\
    N_{x,st} &{}= k_{st}\left(\frac{t_{st}}{b_{st}}\right)^{2}\bar{Et}
\end{array}
\end{equation}
%
Using \verb|align|:
\begin{align}
    N_{x,sk} &= k_{sk}\left(\frac{t_{sk}}{b_{sk}}\right)^{2}\bar{Et}\\
    N_{x,st} &= k_{st}\left(\frac{t_{st}}{b_{st}}\right)^{2}\bar{Et}
\end{align}
%
Using \verb|align| with \verb|\nonumber|:
\begin{align}
    N_{x,sk} &= k_{sk}\left(\frac{t_{sk}}{b_{sk}}\right)^{2}\bar{Et}\nonumber\\
    N_{x,st} &= k_{st}\left(\frac{t_{st}}{b_{st}}\right)^{2}\bar{Et}
\end{align}
\end{document}
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