The good old unit circle! Just about all of trig can be derived from it. For those wondering the unit circle is a circle with a radius of 1 at co-ordinates 0,0 (x,y) on a two dimensional plane.
45 years ago I made A’s in that subject. Never used it after that. Gone from my shrinking brain.
When I think of the math & statistics that I was fluent in (HS & college) that were never touched again and seem like Greek to me now, it’s a little depressing.