Radical with underscript

Question

How can I make the radical end at the same level with 11...1 and 44...4 and put the underbrace below the radical? I have this: \sqrt{ \underbrace{11...1}_n\underbrace{44...4}_{2n}}

Answer 1

Insert the \sqrt with \phantom content and then overlap the \underbraced content using \mathllap (from mathtools):

\documentclass{article}

\usepackage{mathtools}

\begin{document}

\[
  \sqrt{\phantom{11 \cdots 1 \, 44 \cdots 4}}
  \mathllap{\underbrace{11 \cdots 1}_n\underbrace{44 \cdots 4}_{2n}}
\]

\end{document}

Answer 2

Just for the sake of variety, here's a solution that uses \smash[b]{...} and \vphantom to achieve the typographic objective. The \vphantom directive serves to insert what's called a typographic strut, placed outside the square-root term.

\documentclass{article}
\usepackage{amsmath} % for "\smash[b]{...}" macro
\begin{document}
\[
\sqrt{\smash[b]{\underbrace{11...1}_n\underbrace{44...4}_{2n}}} \vphantom{\underbrace{1}_{n}}
\]
\end{document}