Combinatorics

1. Regular odd polygon total intersection point=  https://math.stackexchange.com/questions/1010591/what-is-the-number-of-intersections-of-diagonals-in-a-convex-equilateral-polygon

Problem: https://toph.co/p/irregular-is-tough-regular-is-easy

2. Given two side of a triangle, how many valid possible value of third side- 2*(min(a,b))-1

problem: https://toph.co/p/geometry-forever
n(n−1)(n−2)(n−3)24n(n−1)(n−2)(n−3)24