Hi:
I have Lyx 1.3.1 with Qt interface on Suse 8.1
I typed a multiline equation, then wanted to put a left brace delimiter around it. I tried to select the whole thing, but when I inserted the delimiter it replaced my equation instead of wrapping it on the left side as I expected.
use the split environment, then it is no problem. here is a LaTeX example. You can copy and paste the relevant parts into LyX
Herbert
\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}\begin{multline}
y=\\
f(x) +\\
+g(x)
\end{multline}\[
f: \left(
\begin{split}
y = &&\\
& f(x) +\\
&& +g(x)
\end{split}
\right)
\]
\end{document}