Using align in beamer with overlays

Question

How can I get the align environment to play nicely with beamer overlays? As a simple example I would like

\begin{align*}  
    a = \frac{bc}{c}  
\end{align*}

on the first overlay and then

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

on the second, but I want the equal signs to line up for both. Using beamer's \overprint environment, for example, the equal sign shifts between the two lines I guess because they are of different length.

Answer 1

\pause is the only overlay command that really gives me trouble. I am usually able to workaround with the overlay-aware \only, \uncover, \alt, etc. Just make sure you don't put the alignment character & inside one of their arguments.

In this case we use a standard trick for alignment: make the smaller thing overlap and the larger thing invisible.

\documentclass{beamer}
\usepackage{amsmath}
\usepackage{mathtools}

\begin{document}

\begin{frame}
\begin{align*}
    a &= \mathrlap{\alt<2->{b}{\frac{bc}{c}}}\phantom{\frac{bc}{c}}
\end{align*}
\end{frame}

\end{document}