Equations inside enumerate aligned on item's number

Question

If I type

\begin{enumerate}    
    \item   
    \begin{align*}  
      eqn 1  
      eqn 2  
    \end{align*}  
\end{enumerate}

the label will appear on the first lines and the equations will appear on subsequent lines. How do I make it so that the equations begin on the same line as the enumerate label? Obviosly, I could manually adjust the location using \vspace and \rule, but is there a better way?

Yes, to do the same task without centering, I use \item $\begin{aligned}[t] equations \end{aligned}$ — richtera, Jun 01 '12 at 22:17

score 35 · Accepted Answer · 2014-07-29T06:14:02.990

35

\listfiles
\documentclass{article}
\usepackage{amsmath}
\newcommand\Item[1][]{%
  \ifx\relax#1\relax  \item \else \item[#1] \fi
  \abovedisplayskip=0pt\abovedisplayshortskip=0pt~\vspace*{-\baselineskip}}
\begin{document}

\begin{enumerate}    
\item foo
\Item  
    \begin{align*}  
      eqn 1  \\
      eqn 2  
    \end{align*}  
\Item[a)]  
    \begin{align*}  
      eqn 1  \\
      eqn 2  
    \end{align*}  
\item bar
\end{enumerate}

\end{document}

enter image description here

edited Jul 29 '14 at 06:14

answered Jun 01 '12 at 15:08

This doesn't work for customized item like \Item[<mylabel>]. is it possible fix it? – skpblack Jul 29 '14 at 05:18
1

see myedited answer – Jul 29 '14 at 06:14
@Herbert great job ..... – juanuni May 24 '15 at 06:08
See also http://tex.stackexchange.com/questions/287081/how-to-prevent-a-linebreak-before-align-equation-environment-in-itemize/287091?s=1|0.3679#287091 – John Kormylo Jan 15 '16 at 16:55
1

This affects all equation environments in the Item, not just the first. Is there any way to only remove the top margin on the first equation? – Abraham Murciano Benzadon Apr 05 '21 at 16:50
@AbrahamMurcianoBenzadon not only in that item, but in the whole remaining enumeration – matheburg Sep 30 '23 at 03:24

kiss my armpit · Answer 2 · 2014-07-29T07:37:57.323

It is my personal taste, you might not agree with me. Here is my rule of thumb.

We know that the first line of all items of "list" (enumerate, itemize, etc) are left aligned.
So if the item starts with a multi-line aligned equation, use aligned environment (plus t passed to its optional argument) rather than align*. See my the second item in my example below. Using align* makes the equations centered so it breaks the nature of "list".
But if the item starts with a sentence, use align* environment. See my third item in the given example below. Using align* does not break the nature of "list".

Does it make sense?

\documentclass[preview,border=12pt,12pt]{standalone}% change it back to your own document class
\usepackage{amsmath}

\begin{document}
\begin{enumerate}
% number 1
\item $E\neq mc^2$ is the correct formula.
% number 2
\item
$
\!
\begin{aligned}[t]
 (a+b)^2 
    &= (a+b)(a+b) && \text{by definition} \\
    &= a^2 +ab +ab +b^2 && \text{using FOIL steps}\\
    &= a^2 +2ab +b^2 && \text{simplified}
\end{aligned}
$ 
% number 3
\item Consider the following equations,
\begin{align*}
    \vec \nabla \cdot \vec{B} &= 0\\
    \vec \nabla \times \vec{E} &= -\frac{\partial \vec{B}}{\partial t}
\end{align*}
% number
\item $y=mx+c$ is a street line.
\end{enumerate}
\end{document}

enter image description here

I use align* because it is possible break the alignment across the pages (with \allowdisplaybreaks) unlike aligned that produces unbreakable boxes. — skpblack, Jul 29 '14 at 11:15

Equations inside enumerate aligned on item's number

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