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a rectangle constructed starting with a regular dodecahedron

Source: Brazilian Mathematical Olympiad 1984 P3

July 27, 2018
geometryrectangledodecahedronregular polygon3D geometry

Problem Statement

Given a regular dodecahedron of side aa. Take two pairs of opposite faces: E,EE, E' and F,FF, F'. For the pair E,EE, E' take the line joining the centers of the faces and take points AA and CC on the line each a distance mm outside one of the faces. Similarly, take BB and DD on the line joining the centers of F,FF, F' each a distance mm outside one of the faces. Show that ABCDABCD is a rectangle and find the ratio of its side lengths.