# Numbering an array as one single equation LaTeX

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.comNov 19 '12 at 15:24

This question came from our site for professional and enthusiast programmers.

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
Please consider reading Peter Grill's answer, as use of eqnarray is not recommended. – user2473 Nov 20 '12 at 2:34

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.

## Code:

\documentclass{article}
\usepackage{amsmath}

\begin{document}\noindent
Using \verb|equation| with \verb|array|:
$$\begin{array}{r@{}l} N_{x,sk} &{}= k_{sk}\left(\frac{t_{sk}}{b_{sk}}\right)^{2}\bar{Et}\\ N_{x,st} &{}= k_{st}\left(\frac{t_{st}}{b_{st}}\right)^{2}\bar{Et} \end{array}$$
%
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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What does {r@{}l} exactly mean in \begin{array}{r@{}l} ? – NumberFour Jul 4 '15 at 11:34
@NumberFour: @{} eliminates the inter column spacing that is usually there with tabular and array. Try removing it to see the difference. The r and l set the columns to be left and right aligned. – Peter Grill Jul 4 '15 at 23:26