MathDB
right angle wanted, incircle, circumcircle, perpendicular diagonals

Source: 2020 Croatia MO p3

February 16, 2021
geometryright angleperpendicular

Problem Statement

Given a triangle ABCABC such that AB<ACAB<AC . On sides ABAB and BCBC, points PP and QQ are marked respectively such that the lines AQAQ and CPCP are perpendicular, and the circle inscribed in the triangle ABCABC touches the length PQPQ. The line CPCP intersects the circle circumscribed around the triangle ABCABC at the points CC and TT. If the lines CA,PQCA,PQ and BTBT intersect at one point, prove that the angle CAB\angle CAB is right.