I don't think such a feature exists. You can in theory use spoiler block quotes, which are hidden by default until the user mouse overs them (I'm not sure how they interact on clients that don't have hover states), but they still take up vertical in the post and if you ask me should not be used except for material the person reading the answer is going to want to make a conscious decision to display. That does not include math, so I would very strongly discourage using those just to hide "mathy stuff". That's not what spoiler quote blocks are for.
To render a spoiler block quote, prefix one or more lines with >!
rather than with >
.
It shows up like this.
or with math (yes, I know this math is pretty much nonsensical, I'm just using it to give you an idea of what it'd look like):
$$ \frac{\Delta{}v}{\Delta{}t} = \frac{v_{final} - v_0}{t_{end} - t_{start}} \\ x = (\Delta{}v)^{2} \cdot \epsilon{} $$
Without the spoiler block, that exact same math block is rendered as:
$$ \frac{\Delta{}v}{\Delta{}t} = \frac{v_{final} - v_0}{t_{end} - t_{start}} \\ x = (\Delta{}v)^{2} \cdot \epsilon{} $$