Problem

Source: Iran 3rd round 2009 - final exam problem 5

Tags: Pythagorean Theorem, geometry, combinatorics, Iran



A ball is placed on a plane and a point on the ball is marked. Our goal is to roll the ball on a polygon in the plane in a way that it comes back to where it started and the marked point comes to the top of it. Note that We are not allowed to rotate without moving, but only rolling. Prove that it is possible. Time allowed for this problem was 90 minutes.