Need left large curly bracket that allows alignment in align environment

Question

I know that \begin{cases}\end{cases} can give me large left bracket spanning multiple lines, but it does not work well when I want to align equations inside cases with that outside using &. For instance,

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
x&=1\
y&=2\
&\begin{cases}a&=3\b&=4\end{cases}
\end{align}
\end{document}

The align environment treats cases as a whole object, but I want to align the equal signs both inside and outside of cases. Is there a way to do it?

Answer 1

This is just a hack; hopefully, someone else will find a better answer. Anyways, the inner and outer environments are aligned around =, so you can treat them separately and manually adjust one with something like \hskip. It does look weird with cases environment, so I used aligned instead and inserted a big curly brace with \hbox command.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
x&=1\
y&=2\
&\hskip -.45cm\hbox{$\biggl{$} \begin{aligned}a&=3\b&=4\end{aligned}
\end{align}
\end{document}

Answer 2

By use of the \biggl{ and split environment:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
 x  & = 1   \
 y  & = 2   \
\biggl{\begin{split}
    a & = 3 \
    b & = 4
        \end{split}
\end{align}
\end{document}