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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 .


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.
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