Maths Olympiad Prep

Library / KöMaL problem A /124 of 136

KöMaL problem A, 2023

Geometry Difficulty 8.0 Shortlist Prove it Hungary

The incircle of triangle ABCABC is tangent to sides BCBC, ACAC, and ABAB at points DD, EE, and FF, respectively. Let A′A' denote the point of the incircle for which circle (A′BC)(A'BC) is tangent to the incircle. Define points B′B' and C′C' similarly. Prove that lines A′DA'D, B′EB'E and C′FC'F are concurrent.

We don't reproduce this publisher's solutions. Their own solution is here — work the problem first.
Want a route through all this instead of an archive? The track puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.

Source: KöMaL, licensed Rights held by KöMaL and the MATFUND Foundation. Statement reproduced verbatim; metadata (topic, difficulty) added by this project. Solutions are the publisher's, linked not copied.