答案1
\documentclass{article}
\usepackage{graphicx}
\newcommand\reflectsqrt[2][\relax]{
\ifx\relax#1
\scalebox{1}[-1]{$\sqrt{\scalebox{1}[-1]{$#2$}}$}
\else
\scalebox{1}[-1]{$\sqrt[{\scalebox{1}[-1]{%
\raisebox{-4pt}{$\scriptscriptstyle#1$}}}]{\scalebox{1}[-1]{$#2$}}$}
\fi
}
\begin{document}
$x\reflectsqrt{a}$
$x\reflectsqrt[p]{a_3 + b^3}$
\end{document}
答案2
和stackengine
:
\documentclass{article}
\usepackage{stackengine}
\newlength\rlength
\newcommand\padicroot[2]{\settowidth{\rlength}{\ensuremath{\,#2}}%
\stackMath\stackinset{c}{}{c}{-1.3ex}{\scriptscriptstyle#1}%
{\raisebox{-0.6ex}{\Large$\wedge$}}\mkern-2mu\rlap{\rule[-0.68ex]{\rlength}{0.5pt}}%
\mkern1mu#2}
\begin{document}
\[ \padicroot{p}{y} \]%
\end{document}