Align a text at the left and an equation at the center on the same line within math environment

Question

In the following example, I have some equations within a list. I was wondering how to place each of the equations in items (ii) and (iv) on the same line with their corresponding item number, while centered, and the text beforehand, if any, aligned to the left.

Thanks.

\documentclass{article}
\usepackage{amsmath}
\usepackage{enumerate}
\begin{document}
    \begin{enumerate}[(i)]
        \item Item 1

        \item
        \begin{equation}
            3x + 9y =  -12.
        \end{equation}
        \item Item 3
    \end{enumerate}
\end{document}

This may help with part of your question: Equations inside enumerate aligned on item's number — barbara beeton, May 24 '23 at 01:55
You should also ask yourself if you need both item numbers and equation numbers. In a well-designed document, you should focus on one or the other possibility, but not produce both types of numbering facilities. — Mico, May 24 '23 at 08:57
@Micro: This is just a sample for reference. The list is not intended to number the equations. Yes, I need both numbering types. Thanks. — AEW, May 24 '23 at 17:04

AEW · Accepted Answer · 2023-06-09T18:02:03.277

As I was looking for generality, here is a solution, inferred from the answer of @Werner posted here.

\documentclass{article}
\usepackage{amsmath}
\usepackage{enumerate}
\begin{document}
    \begin{enumerate}[(i)]
        \item Item 1

        \item  \hfill
        $ 3x + 9y =  -12. $
        \hfill\refstepcounter{equation}\textup{(\theequation)}\label{eq1}
        \item Item 3
        \item Equation 4 \hfill
        $ 4x - 5y =  15. $
        \hfill\refstepcounter{equation}\textup{(\theequation)}\label{eq2}

    \end{enumerate}
\end{document}

To make the equations centered, @Werner proposes, in his comment below, the following more elegant solution.

\documentclass{article}
\usepackage{amsmath}
\usepackage{enumerate}
\begin{document}

    \begin{enumerate}[(i)]
        \item Item 1

        \item \hfill
        $ 3x + 9y =  -12. $
        \hfill\llap{\refstepcounter{equation}\textup{(\theequation)}\label{eq1}}
        \item Item 3
        \item \makebox[0pt][l]{Equation 4} \hfill
        $ 4x - 5y =  15. $
        \hfill\llap{\refstepcounter{equation}\textup{(\theequation)}\label{eq2}}

    \end{enumerate} 
\end{document}

If you want the equations centered, then you need to place the equation numbers and any text preceding them inside zero-width boxes. — Werner, Jun 09 '23 at 01:20
@Werner: Could you please show me how to do "zero-width boxes"? — AEW, Jun 09 '23 at 01:26
\rlap makes a zero-width box with a right overlap (similar to \makebox[0pt][l]) while \llap makes a zero-width box with a left overlap (similar to \makebox[0pt][r]). If used immediately after \item you'll need \makebox or insert a \leavevmode before the overlap command. See this code. — Werner, Jun 09 '23 at 05:26

Align a text at the left and an equation at the center on the same line within math environment

1 Answers1