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62nd Ukrainian National Mathematical Olympiad, Third Round, First Tour

Ukraine geometry

Problem

Let be a bisector of triangle . The circle centered at with radius meets the ray at point , and the circle centered at with radius meets the ray at point (points and are different from point ). Prove that . (Mykola Moroz)
Solution
Clearly, triangles and are isosceles, and angles and are vertical (fig. 3). Then . Then as adjacent to equal angles. Also , as is a bisector.

Note that triangles and are similar by two angles. Then , from where . Also note that triangles and are similar by two angles. Then

Techniques

Angle chasingDistance chasing