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69th Belarusian Mathematical Olympiad

Belarus geometry

Problem

A circle of radius is given. A collection of triangles is called good, if the following conditions hold: (i) each triangle from is inscribed in ; (ii) no two triangles from have a common interior point. Determine all positive real numbers such that, for each positive integer there exist a good collection of triangles, each of perimeter greater than .
Solution
1. See IMO-2018 Shortlist, Problem G3.
Final answer
0 < t < 4

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleTrigonometryAngle chasingOptimization in geometry