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Mongolian Mathematical Olympiad

Mongolia geometry

Problem

A circle with center and radius is inscribed in quadrilateral . Another circle with center , () and radius is situated inside the quadrilateral . Circles inscribed in the angles are tangent to the circle at points respectively. If circles tangent to externally and circles tangent to internally and then find the value of .
Solution
By the given condition . Denote ...homothety with center and coefficient . Let denote , , , radius of circles , , , respectively. Then we have If we set then . Similarly, we get and . . If the condition implies .

Consequently and . Thus we conclude that points , , , are lie on a line in this order. Hence we have .
Final answer
R/r

Techniques

HomothetyTangents