Points in Cartesian plane
Source: Brazil National Olympiad 2019 - level 2 - #6
November 23, 2019
Brazilian Math Olympiad
Problem Statement
In the Cartesian plane, all points with both integer coordinates are painted blue. Blue colon
they are said to be mutually visible if the line segment connecting them has no other blue dots. Prove that
There is a set of blue dots that are mutually visible two by two.