MathDB
Proving 4 points concyclic in a right triangle

Source: Canada Junior MO 2023 P2

January 19, 2024
geometryangle bisectorcircumcircle

Problem Statement

An acute triangle is a triangle that has all angles less that 9090^{\circ} (9090^{\circ} is a Right Angle). Let ABCABC be a right-angled triangle with ACB=90.\angle ACB =90^{\circ}. Let CDCD be the altitude from CC to AB,AB, and let EE be the intersection of the angle bisector of ACD\angle ACD with AD.AD. Let EFEF be the altitude from EE to BC.BC. Prove that the circumcircle of BEFBEF passes through the midpoint of CE.CE.