On the toolbar, there's a plus button that gives you a few additional options. One of them is "add spoiler."
It inserts the formula in [], and you have the option to add a title for it. Then you just put whatever you want hidden between the the two formula thingies.
It inserts the formula in [], and you have the option to add a title for it. Then you just put whatever you want hidden between the the two formula thingies.