[Tex/LaTex] amsmath’s intertext and labeling

Question

I have an align environement with intertext:

\usepackage{amsmath,mathtools}
\begin{align*}
x&=f(z)\cos\alpha\\
y&=f(z)\sin\alpha
\shortintertext{with}
f(z)&=\ldots
\end{align*}

However I want a label, centered between the first two equations. So something like this:

\begin{equation}\label{eq:test}
\begin{aligned}
x&=f(z)\cos\alpha\\
y&=f(z)\sin\alpha
\end{aligned}
\end{equation}
with
\begin{equation*}
f(z)=\ldots
\end{equation*}

This allows labeling as I want it, but the third equation is not aligned anymore. Any ideas on how to solve this?

Answer 1

Please always post complete documents,not just fragments.

The split environment is designed for this use:

\documentclass{article}

\usepackage{amsmath,mathtools}
\begin{document}

aaa
\begin{align}
\begin{split}
x&=f(z)\cos\alpha\raisetag{0pt}\\
y&=f(z)\sin\alpha
\end{split}\\
\shortintertext{with}
f(z)&=\ldots
\end{align}

\end{document}