Well, maybe not elementary school, but in highschool you should encounter those, especially second part. Non-degenerate in that case is fancy word for saying th

Well, maybe not elementary school, but in highschool you should encounter those, especially second part. Non-degenerate in that case is fancy word for saying that those point cannot form a line (or a single point) - just that triangle is actually a triangle. This is pretty common term when it comes to math Olympiad so in would be extremely unlikely that someone can get to imo without encountering that in the past during training