MathDB
congruent pairs of lines = 2012 more than not congruent

Source: China Northern MO 2012 p4 CNMO

May 4, 2024
combinatoricscombinatorial geometrygeometry

Problem Statement

There are nn (n4n \ge 4) straight lines on the plane. For two straight lines aa and bb, if there are at least two straight lines among the remaining n2n-2 lines that intersect both straight lines aa and bb, then aa and bb are called a congruent pair of staight lines, otherwise it is called a separated pair of straight lines. If the number of congruent pairs of straight line among nn straight lines is 20122012 more than the number of separated pairs of straight line , find the smallest possible value of nn (the order of the two straight lines in a pair is not counted).