latex amsmath in equation environment

\documentclass{article}
\usepackage{amsmath}
\begin{document}
 some text 
 \begin{equation}
c = \sqrt{a^{2} + b^{2}} text in math
\end{equation}
 now amsmath and text
  \begin{equation}
c = \sqrt{a^{2} + b^{2}} \text{ text in math}
\end{equation}
 more text
\end{document}