Mi Prepa, https://www.facebook.com/photo?fbid=1427344626075511&set=a.575642501245732

 

Mi Prepa, https://www.facebook.com/photo?fbid=1427344626075511&set=a.575642501245732

In the interior of triangle ABC having  there is a point D having . Find .

Proof:

Take E reflection of B across CD, it lies onto CA and D is the circumcenter of ; according to lemma proved at https://stanfulger.blogspot.com/2026/05/problem-lost-source.html,
AE = AD. With  we infer .