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Baltic Way shortlist

Baltic Way geometry

Problem

Let and be two circles with centers and , respectively, with lying on . Let be a common point of and . A line through intersects in and in such that lies between and . The ray intersects in and contains a point such that and lies between and . Show that bisects .

problem


problem
Solution
Let be the second intersection of and . Notice that and since that . It follows that is the reflection of over , thus it suffices to prove that is parallel to , as then is a midline of triangle .

Notice that . Hence by symmetry about , we have

Moreover so we conclude that which proves that and are parallel.



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Alternative solution.

Let be the point on diametrically opposite . As in the first solution, it suffices to prove that is parallel to , as then is a midline of triangle . Note first that so the points , , , and lie on a circle. Since , we see that is the midpoint of the arc of this circle. It follows then that and as we showed in the first solution this implies that , so lines and are parallel.

Techniques

Angle chasingCyclic quadrilaterals