Change appearance of equation numbers locally

Question

In the MWE below, I would like to have the equation numbers in the Chapter "Appendix" (but only there!) being displayed as follows:

(A.1), (A.2), (A.3) ...

\documentclass{book}

\usepackage{amsmath}
\usepackage{mathtools}          
\usepackage[titletoc]{appendix}
\usepackage{titlesec}       
\usepackage{titletoc}
\usepackage{ntheorem}

\newtheorem{prop}{Proposition}

\numberwithin{equation}{chapter}
\numberwithin{prop}{section}

\begin{document}

\chapter{Test Chapter}
\section{XY}

\begin{prop}
\begin{equation}
1+2=3
\end{equation}
\end{prop}

\begin{appendices}

\newtagform{WithA}{(A}{)}              
\usetagform{WithA}                     
\newcommand{\Aref}[1]{(A\ref{#1})}     

\renewcommand\thechapter{}                                          

\makeatletter                                                       
\def\chaptermark#1{\markboth{\MakeUppercase{\@chapapp. \  #1}}{}}   
\makeatother                                                        

\chapter{Additional stuff nobody is interested in}

\titleformat{\section}[runin]                                       
  {\normalfont\normalsize\bfseries}{}{1em}{}                        
\titlespacing*{\section}                                            
  {0pt}{3.25ex plus 1ex minus .2ex}{1em}                            
\titlecontents{section}[1.5em]                                      
  {}{}{}{\titlerule*[7.5pt]{.}\contentspage}                        

\begin{prop}\label{y}
\begin{equation}
      a+b=c
\end{equation}
\end{prop}
Equation (\ref{x}) in Proposition \ref{y} is nice.  
\end{appendices}

\end{document}

EDIT: I have edited my example --> It now produces the desired output, but it doesn't seem to be very elegant! A major drwaback of my solution is that I am bound to forget to use \Aref insted of \ref when referring to equations in the appendix. Hence a solution which does this automatically would be highly appreciated!

Hence, I modify my question to: Are there any better solutions?

Answer 1

Just add

\renewcommand{\theequation}{\Alph{chapter}.\arabic{equation}}

after \begin{appendices}. You'll be able to use \ref or \eqref.

Answer 2

Do it with the appendix package.

\documentclass{book}
\usepackage{amsmath}
\usepackage[toc]{appendix}
\numberwithin{equation}{chapter}
\begin{document}
\chapter{Test Chapter}
\begin{equation}
1+2=3
\end{equation}
\appendix
%\addappheadtotoc
%\appendixpage
\appendixpageoff
\chapter{Appendix}
\begin{equation}
  a+b=c
\end{equation}
\end{document}

For details run texdoc appendix from command prompt.