Let be an acute-angled triangle with circumcircle . Prove that there exists a point with the following property: If is an interior point of , the rays , and intersect the circle again at points and , and the points , and are symmetric to and with respect to the midpoints of the segments and , respectively, then the four points and lie on a common circle.
Solution
We show that the point , usually denoted as , the orthocenter of triangle , has the described property. For this, let be the line through parallel to , and the lines and are defined analogously. No two of the three lines , and are parallel, and therefore, the intersection point of with and the two analogously defined points and exist.
The quadrilateral has right angles at and and is therefore a cyclic quadrilateral. Since the reflection at the midpoint of the segment maps point to and swaps with , it transforms into the circumcircle of the just found cyclic quadrilateral. Consequently, lies on this circle, and is parallel to .
Let the centroid of triangle be . The central dilation with center and factor maps to ; the image of is denoted by . Then, the lines and are parallel, and therefore, lies on the line . Similar arguments can be made by replacing with and , and we learn in total: The three lines , and intersect at .
!
If , the points , and coincide with , and the claim is trivial. From now on, let .
Since , the segment is, by Thales' theorem, a diameter of the circumcircle of the quadrilateral . Again, by Thales' theorem, and therefore also . Thus, the point lies, once again by Thales' theorem, on the circle with diameter . For similar reasons, the points and also lie on this circle, and in particular, we have now found a circle on which all four points , and lie.