How to insert a block in a formula

Question

I am writing a presentation with Beamer and I would like to get the following effect: I have the formula

\begin{align}
a=b 
\end{align}

I would like (after that a=b appears) to put b inside a blue block and put an arrow that starts in the block and goes down and a zero after the arrow (just to say that b converegs to 0)

How should I do?

Answer 1

A rude solution based on other question:

\documentclass{beamer}
\usepackage{amsmath}
\usepackage[most]{tcolorbox}
\tcbset{colback=yellow!10!white, colframe=blue!50!black, 
        highlight math style= {enhanced, %<-- needed for the ’remember’ options
            colframe=blue,colback=blue!10!white,boxsep=0pt}
        }
\begin{document}
\begin{frame}
\begin{align}
a=&\tcbhighmath[remember as=fx]{b}\
\visible<2->{\
&\tcbhighmath[remember, opacityback=0,enhanced jigsaw, boxrule=-0pt, no borderline, colframe=white, overlay={%
\draw[black,very thick,->] (fx.south) to ([yshift=0mm]frame.north);}]{0}
}
\end{align}
\end{frame}
\end{document}

Answer 2

One possible approach using the hf-tikz package:

\documentclass{beamer}
\usepackage[beamer,markings]{hf-tikz}
\usetikzlibrary{calc}
\begin{document}
\begin{frame}
\begin{align}
a=\tikzmarkin<2>{a}b\tikzmarkend{a} 
\end{align}
\begin{tikzpicture}[overlay,remember picture]
  \draw<2>[->,red, thick] (a) ++(0.2,-0.4) -- ++(0,-1)  node[yshift=-8] {0};
\end{tikzpicture}
\end{frame}
\end{document}