Let be an acute triangle such that , with circumscribed circle and circumcenter . Let be the midpoint of and be a point on such that . Let be a point such that is a parallelogram and a point on the same side of as , such that
Let the line intersect at , () and let the circumscribed circle of intersect at point . Prove that the points and are collinear.
Solutions — 2
Solution 1
Let be the symmetric point to in line . Now since , , we have
we have that . Now since we have that are collinear. Note that since
we get that is an isosceles trapezoid.
Since is a parallelogram we have , with being collinear, , and since is an isosceles trapezoid we have and . Since
we have that and are symmetric with respect to the line . Now since and are symmetric with respect to the line as well, this means that is an isosceles trapezoid which means that are concyclic. Since this means that and therefore are collinear.
Solution 2
Denote by the orthocenter of . We use the following well known properties:
(i) Point is the symmetric point of with respect to . Indeed, if is the symmetric point of with respect to then and therefore .
(ii) The symmetric point of with respect to is the point . Indeed, if is the symmetric point of with respect to then is a parallelogram, and since we have .
Since is a parallelogram and we have that is a rectangle. Therefore and implying that and are symmetric with respect to . Denote by the symmetric point of with respect to . Then is an isosceles trapezoid, so is a point on the circumscribed circle of . Moreover and we conclude that . Therefore .
It remains to observe that and and we infer that and are collinear.