Geometry · Circumcircles · Homothety · Inscribed angles · Concyclicity

Problem 5, 2024

← Prev · 61 / 61 · Next →

RegionalProof

A point \(D\) is chosen on the side \(BC\) of a triangle \(ABC\). Points \(E\) and \(F\), both different from \(D\), are chosen on the line \(BC\) so that

\[ BE = BD \qquad \text{and} \qquad CF = CD . \]

The circles circumscribed about the triangles \(ACE\) and \(ABF\) meet for the second time at a point \(G\). Let \(H\) be the centre of the circle circumscribed about the triangle \(EGF\). Prove that the midpoint of the segment \(DH\) is the centre of the circle circumscribed about the triangle \(ABC\).

A B C D E F G
\(E\) and \(F\) lie on the line \(BC\) with \(BE = BD\) and \(CF = CD\); the circles through \(A, C, E\) and through \(A, B, F\) meet again at \(G\).

Sign in to check answers, open hints, read the full solution, and track your progress. Statements are always free.

Serbian Regional Competition (Okruzno takmicenje) 2024, high school grade I, category A, problem 5. Organized by the Mathematical Society of Serbia (DMS). Source