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答案1
\documentclass{article}
\usepackage{stackengine,graphicx,amsmath}
\def\rdots{\rotatebox[origin=l]{29}{$\scriptscriptstyle\ldots\mathstrut$}}
\begin{document}
\stackinset{r}{-2.5pt}{b}{-6pt}{\tiny\rotatebox{29}{$\underbrace{\kern25pt}_k$}}{%
\stackinset{r}{4pt}{t}{-9.0pt}{\tiny\rotatebox{29}{$\overbrace{\kern15pt}^r$}}{%
$a^{b^{c^{d^{\rdots^n}}}}$}}
\end{document}