MathDB
<ALK acute wanted, starting with cyclic ABCD

Source: 2013 Brasil IMO TST 4.2

July 23, 2021
geometryacuteanglescyclic quadrilateral

Problem Statement

Let ABCDABCD be a convex cyclic quadrilateral with AD>BCAD > BC, ABB not being diameter and CDC D belonging to the smallest arc ABAB of the circumcircle. The rays ADAD and BCBC are cut at KK, the diagonals ACAC and BDBD are cut at PP and the line KPKP cuts the side ABAB at point LL. Prove that angle ∠ALK\angle ALK is acute.
Back to ProblemsView on AoPS