One of the excircles of triangle ABC is tangent to the side AB and to the extensions of sides CA and CB at points C1, B1 and A1 respectively. Another excircle is tangent to side AB and to the extensions of sides BA and BC at points B2, C2 and A2 respectively. Line A1B1 and A2B2 intersect at point P,. lines A1C1 and A2C2 intersect at point Q. Prove that the points A, P, Q are collinear
[I]Proposed by S. Berlov geometryexcirclecollinearity