\usepackage{amsmath} % for the equation* environment
\begin{document}
This is a simple math expression \(\sqrt{x^2+1}\) inside text. And this is also the same: 
\begin{math}
\sqrt{x^2+1}
\end{math}
but by using another command.This is a simple math expression without numbering
\[\sqrt{x^2+1}\]  separated from text.
This is also the same:
\begin{displaymath}
\sqrt{x^2+1}
\end{displaymath}
\ldots and this:
\begin{equation*}
\sqrt{x^2+1}
\end{equation*}