Numbering Equations in a Closed Bracket

Question

I want to number equations in a closed bracket as seen in this figure.

I tried fer things but non of them work.

Answer 1

You can use aligned to construct the inner equation alignment and numbering, surrounding it with \left\{...\right\} for the braces:

\documentclass{article}

\usepackage{amsmath}

\makeatletter
% \seteqnum[<optional label>]
\NewDocumentCommand{\seteqnum}{o}{%
  \IfValueTF{#1}
    {\textup{\tagform@{#1}}}% <label> supplied
    {\incr@eqnum \print@eqnum}% <label> not supplied
}
\NewCommandCopy{\ltxlabel}{\ltx@label}
\makeatother

\begin{document}

For example, see~\eqref{eq:last}:
\[
  \left\{
    \begin{aligned}
      a x^2 + b x + c &= d               & \seteqnum \\
              a x + b &= c x^2 + d       & \seteqnum[a] \\
                    a &= b x^2 + c x + d & \seteqnum \ltxlabel{eq:last}
    \end{aligned}
  \right\}
\]

\end{document}

You have to set the equation numbers manually via \seteqnum. The optional argument allows you to insert a numbering of your own. However, such numbering can't be referenced (without more coding). \ltxlabel can be used to label the numbered equations and are then referenceable via \eqref, if needed.

Answer 2

I don't think this is a good way to set down equations, but it's your choice.

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

\usepackage{lipsum} % for filler text

\NewDocumentEnvironment{alignednum}{b}{%
  \sbox0{$\begin{aligned}#1\end{aligned}$}%
  \begin{equation}
  \left\{\,
  \begin{minipage}{\dimeval{\wd0+3em}}
  \setlength{\abovedisplayskip}{0pt}%
  \addtocounter{equation}{-1}%
  \begin{fleqn}
  \noindent
  \begin{align}#1\end{align}
  \end{fleqn}
  \end{minipage}
  \,\right\}
  \notag\end{equation}%
}{\stepcounter{equation}\ignorespacesafterend}

\begin{document}

\lipsum[35][1-4]
\begin{alignednum}
a+b&=c \label{a}\\
d+e&=f \label{b}\\
g+h&=i \label{c}
\end{alignednum}
\lipsum[42][1-4]

\eqref{a}, \eqref{b}, \eqref{c}

\end{document}

How to do it in a more standard way? Compare the following settings.

\documentclass{article}
\usepackage{amsmath,nccmath}
\usepackage{empheq}

\usepackage{lipsum} % for filler text

\NewDocumentEnvironment{alignednum}{b}{%
  \sbox0{$\begin{aligned}#1\end{aligned}$}%
  \begin{equation}
  \left\{\,
  \begin{minipage}{\dimeval{\wd0+3em}}
  \setlength{\abovedisplayskip}{0pt}%
  \addtocounter{equation}{-1}%
  \begin{fleqn}
  \noindent
  \begin{align}#1\end{align}
  \end{fleqn}
  \end{minipage}
  \,\right\}
  \notag\end{equation}%
}{\stepcounter{equation}\ignorespacesafterend}

\begin{document}

\section{Your proposed way}

\lipsum[35][1-4]
\begin{alignednum}
a+b&=c \label{a}\\
d+e&=f \label{b}\\
g+h&=i \label{c}
\end{alignednum}
\lipsum[42][1-4]

\eqref{a}, \eqref{b}, \eqref{c}

\section{A more standard way}

\begin{subequations}\label{global}
\lipsum[57]{1-4}
\begin{empheq}[left=\empheqlbrace]{align}
a+b&=c \label{local-a}\\
d+e&=f \label{local-b}\\
g+h&=i \label{local-c}
\end{empheq}
\lipsum[58][1-2]
\end{subequations}

The system~\eqref{global} is very important; look, in particular, at equation~\eqref{local-b}.

\end{document}