[Tex/LaTex] aligned multi-line equation with intertext and block-wise numbering

alignequationsnumbering

I try to reproduce the output that is generated by:

\begin{equation}
    \begin{aligned}
    (x-x_1)^2 + (y-y_1)^2 & = r_1^2 \\
    (x-x_2)^2 + (y-y_2)^2 & = r_2^2 \\
    (x-x_3)^2 + (y-y_3)^2 & = r_3^2 \\
    \end{aligned}
    \label{label a}
\end{equation}
Additional lines:
\begin{equation}
    \begin{aligned}
    (x-x_k)^2 + (y-y_k)^2 & = r_k^2     
    \end{aligned}
    \label{label b}
\end{equation}

but make the two aligned blocks use the same alignment (as if it was just one align block.
I tried to do this with just one align block + intertext for the line of text in the middle. But this gave me 4 lines of equation with 4 individual numbers which is not desired.

Thanks for your help and sorry for my bad english.

Best Answer

Like this?

\documentclass{article}
\usepackage{mathtools}
\begin{document}
  \begin{align}
    (x-x_1)^2 + (y-y_1)^2 & = r_1^2 \notag\\
    (x-x_2)^2 + (y-y_2)^2 & = r_2^2     \label{label a}\\
    (x-x_3)^2 + (y-y_3)^2 & = r_3^2 \notag
\shortintertext{Additional lines:}
    (x-x_k)^2 + (y-y_k)^2 & = r_k^2
    \label{label b}
\end{align}
\end{document}

enter image description here

\shortintertext is from mathtools package and gives a tighter spacing than \intertext.

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