# How to make all the exponents same size?

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$a^9 \cdot a^{99} \cdot a^{999} \cdot a^{9999} \cdots a^{\overbrace{999 \cdots 9}^{\text{10 digits}}}=a^x$
\end{document}

• Your problem is not very clear. Could you be more explicit? – Bernard Feb 2 at 0:29
• Use \scriptstyle: \documentclass{article} \usepackage{amsmath} \begin{document} $a^9 \cdot a^{99} \cdot a^{999} \cdot a^{9999} \cdots a^{\overbrace{\scriptstyle 999 \cdots 9}^{\text{10 digits}}}=a^x$ \end{document}. And please do not use $$...$$ but $...$ as in the above example. – Schrödinger's cat Feb 2 at 0:37

I recommend not using \underbrace and \overbrace directly, because they can have bad effects on spacing.

Here's a version that changes sizes when in subscripts or superscripts.

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand{\oubr@demote}[1]{%
\ifx#1\displaystyle\scriptstyle\else
\ifx#1\textstyle\scriptstyle\else
\scriptscriptstyle\fi\fi
}
\newcommand{\obr}[2]{{\mathpalette\oubr@over{{#1}{#2}}}}
\newcommand{\oubr@over}[2]{\oubr@@over#1#2}
\newcommand{\oubr@@over}[3]{\overbrace{#1#2}^{\oubr@demote#1#3}}
\newcommand{\ubr}[2]{{\mathpalette\oubr@under{{#1}{#2}}}}
\newcommand{\oubr@under}[2]{\oubr@@under#1#2}
\newcommand{\oubr@@under}[3]{\underbrace{#1#2}_{\oubr@demote#1#3}}
\makeatother

\begin{document}

$\ubr{a^9 \cdot a^{99} \cdot a^{999} \cdot a^{9999} \cdots a^{\obr{999 \cdots 9}{\text{10 digits}}}} {\text{umpteen times}}$

\end{document}