Problem

Source: 2002 China National Olmpiad

Tags: combinatorics proposed, combinatorics



Suppose that a point in the plane is called good if it has rational coordinates. Prove that all good points can be divided into three sets satisfying: 1) If the centre of the circle is good, then there are three points in the circle from each of the three sets. 2) There are no three collinear points that are from each of the three sets.