In the acute isosceles triangle ABC the altitudes BB1 and CC1 are drawn, which intersect at the point H. Let L1 and L2 be the feet of the angle bisectors of the triangles B1AC1 and B1HC1 drawn from vertices A and H, respectively. The circumscribed circles of triangles AHL1 and AHL2 intersects the line B1C1 for the second time at points P and Q, respectively. Prove that points B,C,P and Q lie on the same circle.(M. Plotnikov, D. Hilko) geometryConcyclicisoscelesorthocenter