Shortlist 2017/G8
Source:
July 10, 2018
geometryIMO Shortlist
Problem Statement
There are mutually external circles drawn on a blackboard, such that no two are tangent and no three share a common tangent. A tangent segment is a line segment that is a common tangent to two circles, starting at one tangent point and ending at the other one. Luciano is drawing tangent segments on the blackboard, one at a time, so that no tangent segment intersects any other circles or previously drawn tangent segments. Luciano keeps drawing tangent segments until no more can be drawn.Find all possible numbers of tangent segments when Luciano stops drawing.