IMO 2019 Problem 2
IMO 2019 Problem 2 is a plane geometry problem from the International Mathematical Olympiad 2019, Problem 2: four concyclic points on two cevians.
Problem
In triangle $ABC$, point $A_1$ lies on side $BC$ and point $B_1$ lies on side $AC$. Let $P$ and $Q$ be points on segments $AA_1$ and $BB_1$, respectively, such that $PQ$ is parallel to $AB$. Let $P_1$ be a point on line $PB_1$, such that $B_1$ lies strictly between $P$ and $P_1$, and $\angle PP_1C = \angle BAC$. Similarly, let $Q_1$ be a point on line $QA_1$, such that $A_1$ lies strictly between $Q$ and $Q_1$, and $\angle CQ_1Q = \angle CBA$. Prove that points $P$, $Q$, $P_1$, and $Q_1$ are concyclic.
Source International Mathematical Olympiad 2019, Problem 2
The diagram
Drag any point and the whole construction follows; ▶ runs a point along its segment and back. Tap a point to pin it, double-click to go back to the start. Effects redraws the figure in grayscale for print, in colour, or with the givens in focus.