MathDB
convex polygon and quadrilaterals

Source: 2021ChinaTST test3 day1 P1

April 13, 2021
combinatorial geometrycombinatoricsgraph theory

Problem Statement

Given positive integer n5 n \ge 5 and a convex polygon PP, namely A1A2...An A_1A_2...A_n . No diagonals of PP are concurrent. Proof that it is possible to choose a point inside every quadrilateral AiAjAkAl(1i<j<k<ln) A_iA_jA_kA_l (1\le i<j<k<l\le n) not on diagonals of PP, such that the (n4) \tbinom{n}{4} points chosen are distinct, and any segment connecting these points intersect with some diagonal of P.