The WYSIWYG toolbar in the body field editor includes a "mathjax" button with a sigma (Σ) icon. Clicking that button brings up a dialog where you can enter raw TeX (LaTeX), and the inserted formula will render properly in the WYSIWYG preview and on the published page. Example: \(x=\frac{1+y}{1+2z^2}\)
By default, the formulas are inserted inline. Single dollar sign, square bracket, and parenthesis delimiters are not supported. Double dollar sign delimiters render on the published page, but not in the editor; however, you can add a formula to its own centered line to get the same effect as $$.
\(|x| = \left\{ \begin{array}{rl} -x &\mbox{ if $x<0$} \\ x &\mbox{ otherwise} \end{array} \right.\)
This feature uses the MathJax JavaScript library to render mathematics that use CSS with web fonts in modern web browsers, with graceful fallbacks for older browsers.