TOT 2004 Spring - Junior A-Level p5 polygonal billiard table
Source:
February 25, 2020
combinatorics
Problem Statement
All sides of a polygonal billiard table are in one of two perpendicular directions. A tiny billiard ball rolls out of the vertex of an inner angle and moves inside the billiard table, bouncing off its sides according to the law “angle of reflection equals angle of incidence”. If the ball passes a vertex, it will drop in and srays there. Prove that the ball will never return to .