# Shrink equations horizontally to fit surrounding tcolorbox, compatible with align* commands

I'd like to put frames around equations in my beamer slides.

For this purpose, I use the package tcolorbox.

I use the macro \fiteq{} that I found somewhere else on this site to shrink overlong equations horizontally (see first equation below).

How can I adopt this macro to work with align commands, such as \\ or &? (See second equation below. When I surround the content with \fiteq{} I get error messages)

I would be most happy with a solution that shrinks the equations instead of expanding the surrounding box.

Also: I would also like to be able to use the equation environment instead of align*, and then use \\ and & in a split environment. However, replacing ams align* by ams equation* yields bad vertical alignment:

The code for the first image is below. For the second image I replaced ams align* by ams equation* in the first tcolorbox.

\documentclass{beamer}
\usepackage[most]{tcolorbox}
\newcommand\fiteq[1]{%
\sbox{\mybox}{$\displaystyle#1$}%
\ifdim\wd\mybox>.85\textwidth\resizebox{.85\textwidth}{!}{\usebox{\mybox}}%
\else\usebox{\mybox}\fi%
}
\newsavebox{\mybox}
\begin{document}
\begin{frame}
\begin{tcolorbox}[ams align*]% Replace by ams equation* to get second image
\fiteq{
A=B=C=D=E=F=G=H=I=J=K=L=M=N=O=P=Q=R=S=T=U=V=W=X=Y=Z
}
\end{tcolorbox}
\begin{tcolorbox}[ams align*]
&A=B=C=D=E=F=G=H=I=J=K=L=M=N=O=P=Q=R=S=T=U=V=W=X=Y=Z\\
\Leftrightarrow &A=B=C=D=E=F=G=H=I=J=K=L=M=N=O=P=Q=R=S=T=U=V=W=X=Y=Z
\end{tcolorbox}
\end{frame}
\end{document}


if you change align* to equation*, you can embed either split or aligned within it, and \fiteq does what is wanted. it's also good to nullify \abovedisplayskip to avoid a large gap above the formula.

\documentclass{beamer}
\usepackage[most]{tcolorbox}
\newcommand\fiteq[1]{%
\sbox{\mybox}{$\displaystyle#1$}%
\ifdim\wd\mybox>.85\textwidth\resizebox{.85\textwidth}{!}{\usebox{\mybox}}%
\else\usebox{\mybox}\fi%
}
\newsavebox{\mybox}
\begin{document}
\begin{frame}
\begin{tcolorbox}[ams align*]% Replace by ams equation* to get second image
\fiteq{
A=B=C=D=E=F=G=H=I=J=K=L=M=N=O=P=Q=R=S=T=U=V=W=X=Y=Z
}
\end{tcolorbox}
\begin{tcolorbox}[ams equation*]
\abovedisplayskip=0pt \relax
\fiteq{\begin{split}
&A=B=C=D=E=F=G=H=I=J=K=L=M=N=O=P=Q=R=S=T=U=V=W=X=Y=Z\\
\Leftrightarrow &A=B=C=D=E=F=G=H=I=J=K=L=M=N=O=P=Q=R=S=T=U=V=W=X=Y=Z
\end{split}}
\end{tcolorbox}
\end{frame}
\end{document}