Equation tags for parts of alignments

Question

Consider the following example:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
  \[
  1+2=3
  \implies
  \left\{
    \begin{aligned}
    &a+b=c, \\
    &x=y+z.
    \end{aligned}
  \right.
  \]
\end{document}

Is there a way to set up two equation tags, one per line in the aligned environment?

Answer 1

You can do this by using the empheq package from the mh bundle. See code below for an example.

\documentclass[11pt,a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}       % loads »amsmath«
\usepackage{empheq}

\begin{document}
  \begin{empheq}[left={1+2=3\implies\empheqlbrace}]{align}
    a+b &= c \\
    x &= y+z
  \end{empheq}
\end{document}

Answer 2

Just a different way of obtaining the same result:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align}
  \smash{
      \raisebox{\dimexpr-.5\normalbaselineskip-.5\jot}
          {$1+2=3\implies\left\{\begin{array}{@{}c@{}}\null\\\null\end{array}\right.$}}
    a+b &= c, \\
  x &= y+z.
\end{align}
\end{document}

The key here is to set the "left-hand side" separately from the "right-hand side" and align the two using some vertical movement.