Let be an acute-angled triangle, with . Let be the midpoint of side , and , be the feet of the altitudes from , , respectively. Denote by , the midpoints of segments , , respectively. Suppose is a point on the line such that .
Prove that .
Solution
Without loss of generality, assume . Construct the circumcircle of .
Lemma 1. The line , the line , and the line are all tangent to circle .

Proof. Note that , both lie on the circle with diameter , whose center is . Hence . Therefore
So by the tangent-chord angle property, is tangent to . By symmetry, is also tangent to .
Moreover, , so is also tangent to . ♡
Now consider the circle , together with the circle centered at with radius . Since , and is tangent to while is tangent to , lies on the radical axis of and . Similarly, so does . In other words, the line is exactly the radical axis of and , and lies on this radical axis. Hence, considering the power of with respect to these two circles: , which gives . This completes the proof.