Inside the post toolbox, click the Formula button. You can toggle between the Live Editor, where you can type out formulas via a virtual keyboard, and the Basic Editor, where you can see and enter the code that's generating the formula.
Code Editor Demonstration
To open the code editor, click the </> icon in the rich text editor. Then, type your code in the black box. The code box will automatically pretty-print your code.
Open the Draw editor by selecting the Draw button in the Post Toolbox.
The Draw editor contains an open canvas. You can draw inside the canvas just as you would in Microsoft Paint. For best results, use a stylus.
You can annotate formulas inside posts and comments. For the demonstration below, we commented on an incorrect formulation of the Bayes' theorem by taking a screenshot of the equation, opening the comment editor, selecting the Draw tool, uploading the screenshot to the canvas background, and annotating the canvas.