1

Here is the MWE:

\documentclass{beamer}
\usepackage{amsmath}
\begin{document}
\setbeamertemplate{navigation symbols}{}
\begin{frame}[shrink]
  \begin{alignat*}{5}
    \uncover<+->{t_n &= a + (n-1)d  \\}
    \uncover<+->{t_3 &= a + (3-1)d \tag{1} \\}
    \cline{1-1}
    \uncover<+->{t_3 &= a + (3-1)d  \\}
    \uncover<+->{t_3 &= a + (3-1)d  \\}
    \uncover<+->{-20 &= -20 \tag{2}\\}
    \notag
  \end{alignat*}
  \vskip-1.5em
\end{frame}
\end{document}

With \cline, it produces a cryptic error:

! Misplaced \omit.
\@cline #1-#2\@nil ->\omit 
                           \@multicnt #1\advance \@multispan \m@ne \ifnum \@...
l.16 \end{frame}

How do I get a simple horizontal line in the place indicated? It would be great if it can have an overlay.

3

Do you want this?

\documentclass{beamer}
\usepackage{amsmath}
\begin{document}
\setbeamertemplate{navigation symbols}{}
\begin{frame}[shrink]
  \begin{alignat*}{5}
    \uncover<+->{t_n &= a + (n-1)d  \\}
    \uncover<+->{t_3 &= a + (3-1)d \tag{1} \\\cline{1-1}}
    \uncover<+->{t_3 &= a + (3-1)d  \\}
    \uncover<+->{t_3 &= a + (3-1)d  \\}
    \uncover<+->{-20 &= -20 \tag{2}\\}
    \notag
  \end{alignat*}
  \vskip-1.5em
\end{frame}
\end{document}

Edit: Or this?

\documentclass{beamer}
\usepackage{amsmath}
\begin{document}
\setbeamertemplate{navigation symbols}{}
\begin{frame}[shrink]
  \begin{alignat*}{5}
    \uncover<+->{t_n &= a + (n-1)d  \\}
    \uncover<+->{t_3 &= a + (3-1)d \tag{1}}
    \uncover<1->{ \\\cline{1-1}}
    \uncover<+->{t_3 &= a + (3-1)d  \\}
    \uncover<+->{t_3 &= a + (3-1)d  \\}
    \uncover<+->{-20 &= -20 \tag{2}\\}
    \notag
  \end{alignat*}
  \vskip-1.5em
\end{frame}
\end{document}

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