The Pythagorean theorem is pretty important. You should be able to represent it graphically because it shows that you are a thinking creature to the point of math.

Works also if you have no other common basis for communication, not even mimics. That might well save you from a live autopsy in case you get abducted by aliens because it would make them curious.

Most people don't need to themseves apply trigonometry outside school even once in their entire life.

You can show the transformation of a boolean conjunction to a disjunction applying De Morgan's law. That is a much less common skill and actually useful to much more devs than trigonometry. It might also be something the previous human didn't already show.

But obviously, you get much better chances of survival in case of being abducted by aliens if you can draw cute Cthulhu's. So better learn that and always have some paper and a pencil on you.