To make the math in beamer look like the usual LaTeX math, you only have to insert the following declaration in your preamble \usefonttheme[onlymath]{serif}.

The following illustration is obtained with the standard Beamer font:

The following code corresponds generates the same content as above but with changing Math equations font style:

% Change Math font style in Beamer
\documentclass{beamer}

% Theme choice
\usetheme{AnnArbor}

% Change Math font
\usefonttheme[onlymath]{serif}

\begin{document}

\begin{frame}{Change Math font style}

\begin{block}{Pythagoras' theorem}
If you square the two shorter sides in a right-angled triangle and add them together, you get the same as when you square the longest side (the hypotenuse):
$x^2 + y^2 = z^2$
where $x$ and $y$ are the two shorter sides and $z$ is the hypotenuse.
\end{block}

\textbf{Equations in Beamer: }
\begin{align*}
f(x) &= x^2\\
g(x) &= \frac{1}{x}\\
F(x) &= \int^a_b \frac{1}{3}x^3
\end{align*}

\end{frame}

\end{document}

Here is the obtained results:

The declaration \usefonttheme[onlymath]{serif} changes the math font of the beamer theme to serif. You can also change the font of the base text and make different combinations tuning both. Here you can find a nice list of already compiled combinations of different text and mathematical fonts.