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a given triangle is divided into n triangles ... tiling triangles related

Source: Spanish Mathematical Olympiad 1987 P3

August 2, 2018
combinatorial geometryTilingtilingsTrianglecombinatorics

Problem Statement

A given triangle is divided into nn triangles in such a way that any line segment which is a side of a tiling triangle is either a side of another tiling triangle or a side of the given triangle. Let ss be the total number of sides and vv be the total number of vertices of the tiling triangles (counted without multiplicity). (a) Show that if nn is odd then such divisions are possible, but each of them has the same number vv of vertices and the same number ss of sides. Express vv and ss as functions of nn. (b) Show that, for nn even, no such tiling is possible