Matrices in equations

Question

I would like to know how it is possible to put matrices and determinants inside equations. In particular I would like to know how to generate the following equality:

Thanks in advanced!

Answer 1

The amsmath package provides an environment called pmatrix -- short for "matrix surrounded by (round) parentheses", I suppose -- which is perfectly suited for the job.

\documentclass{beamer}
\usepackage{amsmath,amssymb}
\begin{document}
\begin{frame}
\[
\det
\begin{pmatrix}
   \lambda & -1 \\
   q & \lambda+p
\end{pmatrix}
=0
\quad \Leftrightarrow \quad
\lambda^2+p\lambda+q=0
\]
\end{frame}
\end{document}

Answer 2

If you want just one part of your formula using the article class, it is possible, for example to use, sansmath package, within the enviroment,

\begin{sansmath} 
....
....
\end{sansmath}

Here there is a small example,

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\usepackage{sansmath}
\begin{document}
This text it is written with the Computer Modern font.
\begin{sansmath} 
\[\det
\begin{pmatrix}
\lambda & -1\\
q & \lambda+p
\end{pmatrix}
=0 \iff \lambda^2+p\lambda+q=0
\]
\end{sansmath}
After you have the same initial font.
\end{document}