Problem

Source: Czech-Polish-Slovak Match, 2009

Tags: geometry, geometric transformation, reflection, parallelogram, geometry unsolved



Given a circle, let $AB$ be a chord that is not a diameter, and let $C$ be a point on the longer arc $AB$. Let $K$ and $L$ denote the reflections of $A$ and $B$, respectively, about lines $BC$ and $AC$, respectively. Prove that the distance between the midpoint of $AB$ and the midpoint of $KL$ is independent of the choice of $C$.