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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)
(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