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Bulgarian Winter Tournament

Bulgaria geometry

Problem

The inscribed circle in () is tangent to its sides , , and at points , , and , respectively. Let be the foot of the perpendicular from to (). If the circles circumscribed about and intersect for the second time at point , prove that .

(Stoyan Boev)
Solution
From the fact that lies on the circle circumscribed about and it follows that is an exterior bisector of and it remains to prove that is a bisector of , i.e. . From and it follows that , i.e. Let be the center of the circle inscribed in and the line intersects for the second time at point . Then and analogously , i.e. . But , , i.e. and are corresponding elements in similar triangles and , which completes the proof.

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleCyclic quadrilateralsTangentsAngle chasing