Problem:
Let be a fixed triangle with , and let be a variable point on arc of its circumcircle. Let be the incenter of and the altitude from . The circumcircle of intersects lines and again at and . Finally, let be the projection of onto line . Prove that the line through and the midpoint of passes through a fixed point as varies.
, 2017
Solution
Solution:
Let be the midpoint of arc not containing and let be the point where the incircle of triangle touches . Also let be the projection from to . We claim that is the desired fixed point.
By Simson's Theorem on triangle and point we have that points are collinear and since quadrilateral is a rectangle we have that line passes through the midpoint of . Thus it suffices to show that lies on line .
Now, let be the -excenters of triangle respectively. Then is the orthocenter of triangle and is the Cevian triangle of with respect to triangle . It's also well-known that is the midpoint of .
Let be the reflection of over and let be the reflection of over . Clearly is the midpoint of . If we could prove that were collinear then by taking a homothety centered at with ratio we would have that points were collinear as desired. Thus it suffices to show that points are collinear.
Let lines and intersect at and let lines and intersect at . Then it's well-known that ( ) is harmonic and projecting from we have that is harmonic. But which means that bisects angle . But it's clear by the definition of that bisects angle which implies that points are collinear as desired. This completes the proof.