Problem:
Let be an acute-angled triangle. A circle passes through points and and meets the sides and at points and , respectively. Let be the circumcircle of the triangle . Prove that is equal to the circumradius of the triangle .

Problem:
Let be an acute-angled triangle. A circle passes through points and and meets the sides and at points and , respectively. Let be the circumcircle of the triangle . Prove that is equal to the circumradius of the triangle .

Solution:
Recall that, in every triangle, the altitude and the diameter of the circumcircle drawn from the same vertex are isogonal. The proof offers no difficulty, being a simple angle chasing around the circumcircle of the triangle.
Let be the circumcenter of the triangle . From the above, one has . On the other hand , for is cyclic. Thus , implying that in the triangle cevians and are isogonal. So, since is a radius of the circumcircle of triangle , one obtains that is an altitude in this triangle.
Moreover, since is the perpendicular bisector of the line segment , one has , and furthermore .
Chord is common to and , hence . It follows that , so is a parallelogram. The conclusion is now obvious.