4 open half-planes cover the plane
Source: 1951 Hungary - Kürschák Competition p3
October 10, 2022
combinatoricscombinatorial geometry
Problem Statement
An open half-plane is the set of all points lying to one side of a line, but excluding the points on the line itself. If four open half-planes cover the plane, show that one can select three of them which still cover the plane.