I am working on a Beamer presentation, and I would like to use sans serif font (CMBright) for text, and a serif font (Computer Modern) for maths. Sans serif maths looks pretty ugly to me.
Beamer default settings are sans serif maths and text :

I found I can switch maths font to serif with the line
\usefonttheme[onlymath]{serif}
which yields (see MWE1 below)

Now maths look nice, but I would like to change the default text font to CMBright. I tried to naively combine those two lines (see MWE2)
\usefonttheme[onlymath]{serif}
\usepackage{cmbright}
but is gives me everything in CBMright (ie sans serif)

I seemed to find some people using LuaTeX or related systems, but I would like to find a solution using PDFLaTeX only.
More generally, is there a way to set the maths and text fonts independently using PDFLaTeX, ie to any font different than CM and CMBright ?
MWE1
\documentclass{beamer}
\usefonttheme[onlymath]{serif}
\begin{document}
\begin{frame}
Some text, $x^2 + y^2 = r^2$
\end{frame}
\end{document}
MWE2
\documentclass{beamer}
\usepackage{cmbright}
\begin{document}
\begin{frame}
Some text, $x^2 + y^2 = r^2$
\end{frame}
\end{document}
