# How to insert an un-aligned equation inside an aligned equation block?

Here is a block of LaTeX that contains an un-aligned equation between 2 aligned equation blocks.

\begin{gather*}
\begin{align*}
a &= b (c + d)&\\
&= bc + bd&
\end{align*}\\
[ \text{as } bc = zx \text{, and } bd = zy \text{, (this may be a long description occupying one line)} ]\\
\begin{align*}
&= zx + zy&\\
&= z(x + y)&
\end{align*}\\
\end{gather*}


The output looks like :

           a = b(c + d)
= bc + bd
[as bc = zx, and bd = zy, (this may be a long description occupying one line)]
= zx + zy
= z(x + y)


You should notice that the "equal" sign in the second aligned block is not placed in the same position as the first aligned block.

I want to know that is there a way to align the "equal" sign in the second aligned block so that it is placed at the same position as the first aligned block. i.e. It should look like :

           a = b(c + d)
= bc + bd
[as bc = zx, and bd = zy, (this may be a long description occupying one line)]
= zx + zy
= z(x + y)


Thanks for any suggestion.

You can use \intertext to add a line of text between aligned blocks.

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{gather*}
\begin{align*}
a &= b (c + d)&\\
&= bc + bd&
\intertext{as $bc = zx$, and  $bd = zy$, (this may be a long description occupying one line)}
&= zx + zy&\\
&= z(x + y)&
\end{align*}\\
\end{gather*}
\end{document} • but the un-aligned part may be a long and complicated mathematical equation. – user1129812 Sep 8 '14 at 6:35
• If it is a long complicated mathematical expression that does not fit on a lintel line, probably you should write in in a different way, not interwinding the two. – Guido Sep 8 '14 at 6:38
• \intertext{} is a text mode environment. It seems a bit difficult to align content if we turn it into "displaystyle" math mode using . However, "textstyle" math equation content seems align OK. Perhaps it may be a solution for not too complicated inter equation. – user1129812 Sep 8 '14 at 7:40
• If you have to interleave a displayed formula, how do you know that the block below is not a continuation of the interleaved formula? Also if is very long, the alignment can be in a different page as well. This is why I suggested to present the computation in a different way. – Guido Sep 8 '14 at 7:47
• It seems that we could insert display style math by using \intertext{e.g. $\displaystyle \sum_{n=1}^m x^2$}. Then this would solve my problem. Thanks. – user1129812 Sep 8 '14 at 8:09