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Constraints:

I have to use top aligned aligned to let the baseline of the top equation be aligned with the enumerate label.

Issues:

The curly brace goes beyond the top equation so it does not look good.

Questions:

How to prevent the opening curly brace from going out over the top equation?

\documentclass[preview,border=12pt,12pt,varwidth]{standalone}
\usepackage{mathtools,enumitem}

\begin{document}
Find the solution of
\begin{enumerate}[label=\alph*.]
    \item 
    $\!
    \left\{
    \begin{aligned}[t]
        2x +5y &=12\\
        3x -8y &=11
    \end{aligned}
    \right.
    $
    \item 
    $\!
    \left\{
    \begin{aligned}[t]
        -2x -3y &=7\\
        x -2y &=1
    \end{aligned}
    \right.
    $
\end{enumerate}
\end{document}

enter image description here

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Is adjustbox useful to fix this problem? I haven't tried it yet. –  cyanide-based food May 16 at 4:45
    
You may use the \cropdelims macro defined in this answer to achieve this effect. (\item $\!\cropdelims \left\{ {\begin{aligned}[t] 2x +5y &=12\\ 3x -8y &=11 \end{aligned} } \right.$) –  Paul Gessler May 16 at 5:03
    
It seems very much sufficient to use \item \raisebox{-.5\dimexpr\jot+\baselineskip}{$ \left\{\begin{array}{r@{}l} 2x + 5y & {}= 12 \\[\jot] 3x - 8y & {}= 11 \end{array}\right. $} –  Werner May 16 at 5:40
    
@Werner: Is the calculation based on an exact dimension arithmetic or an approximation? –  cyanide-based food May 16 at 5:44
    
In my honest opinion, i like it better, if a. and b. would be at the tip of the opening brace. This way one could also use easily the cases environment. –  Ronny May 16 at 6:04

4 Answers 4

up vote 4 down vote accepted

Use delarray:

\documentclass{article}
\usepackage{amsmath,enumitem,array,delarray}

\newenvironment{system}[1][c]
 {\begin{array}[#1]\{{@{} r @{} >{{}} l @{}}.}
 {\end{array}}

\begin{document}
Find the solution of
\begin{enumerate}[label=\alph*.]
\item $\begin{system}[t]
       2x + 5y & = 12 \\[\jot]
       3x - 8y & = 11
      \end{system}$

\item $\begin{system}[t]
       -2x -3y & = 7 \\[\jot]
         x -2y & = 1
      \end{system}$
\end{enumerate}
\end{document}

enter image description here

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enter image description here

\documentclass[preview,border=12pt,12pt,varwidth]{standalone}
\usepackage{mathtools,enumitem,calc}

\newcommand{\Bob}[1][2em]{%
    \raisebox{-.5\height}{$\left\{\rule[-#1/2]{0pt}{#1}\right.$}
    }

\begin{document}
Find the solution of
\begin{enumerate}[label=\alph*.]
    \item 
    $\!\Bob
    \begin{aligned}[t]
        2x +5y &=12\\
        3x -8y &=11
    \end{aligned}
    $
    \item 
    $\!\Bob
        \begin{aligned}[t]
        -2x -3y &=7\\
        x -2y &=1
    \end{aligned}\quad\text{ la suite}
    $
\end{enumerate}
\end{document}
share|improve this answer
    
Yes of course it compiles but you broke the rule of the game. –  cyanide-based food May 16 at 4:15
    
@MoneyOrientedProgrammer Is it better ? –  Tarass May 16 at 4:46
    
Yes. Better but not the best. :-) –  cyanide-based food May 16 at 4:54
    
I missed again something ? –  Tarass May 16 at 5:25
    
Also interesting: What's Bob doing and how does it work; can you extend your answer on that? That would be quite nice –  Ronny May 16 at 5:30

Here's a fairly intuitive and informal way of achieving your requirement:

enter image description here

\documentclass{article}
\usepackage{enumitem}

\newcommand{\twolinebrace}{%
  \raisebox{-.5\dimexpr\jot+\baselineskip}{$
    \left\{\begin{array}{@{}c@{}}\null\\[\jot]\null\end{array}\right.\kern-\nulldelimiterspace$}}

\begin{document}
Find the solution of
\begin{enumerate}[label=\alph*.]
  \item 
    $\twolinebrace
    \begin{array}[t]{r@{}l}
      2x + 5y & {}= 12 \\[\jot]
      3x - 8y & {}= 11
    \end{array}$

  \item 
    $\twolinebrace
    \begin{array}[t]{r@{}l}
      -2x -3y & {}= 7 \\[\jot]
        x -2y & {}= 1
    \end{array}$
\end{enumerate}
\end{document}

The idea is to lower the brace rather than the entire construction. And, perhaps even simpler:

enter image description here

\documentclass{article}
\usepackage{amsmath,enumitem}

\begin{document}
Find the solution of
\begin{enumerate}[label=\alph*.]
  \item 
    $\raisebox{-.5\height}{$\biggl\{$}
    \begin{array}[t]{r@{}l}
      2x + 5y & {}= 12 \\[\jot]
      3x - 8y & {}= 11
    \end{array}$

  \item 
    $\raisebox{-.5\height}{$\biggl\{$}
    \begin{array}[t]{r@{}l}
      -2x -3y & {}= 7 \\[\jot]
        x -2y & {}= 1
    \end{array}$
\end{enumerate}
\end{document}
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Here, I use a \scalerel[max-width]{\{}{...} in lieu of the \left\{...\right. syntax to match the delimiter to the height of the item.

\documentclass[preview,border=12pt,12pt,varwidth]{standalone}
\usepackage{mathtools,enumitem}
\usepackage{scalerel}
\begin{document}
Find the solution of
\begin{enumerate}[label=\alph*.]
    \item 
    $\!
    \scalerel[1.7ex]{\{}{%
    \begin{aligned}[t]
        2x +5y &=12\\
        3x -8y &=11
    \end{aligned}
    }
    $
    \item 
    $\!
    \scalerel[1.7ex]{\{}{%
    \begin{aligned}[t]
        -2x -3y &=7\\
        x -2y &=1
    \end{aligned}
    }
    $
\end{enumerate}
\end{document}

enter image description here

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