\setlength{\fboxrule}{0.01pt}
\setlength{\fboxsep}{0pt}
Texte \fbox{\phantom{à trou}} simple
\[    \sqrt{\boxed{\frac{1}{200}}}
\quad \sqrt{\boxed{}}
\quad \sqrt{\boxed{\phantom{\frac{1}{200}}}}
\quad \sqrt{\boxed{\vphantom{\frac{1}{200}}}}
\quad \sqrt{\boxed{\hphantom{\frac{1}{200}}}}
\]