Concyclic Points Associated with an Angle Bisector and an Excircle
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Let ABC be a triangle with incenter I and circumcenter O, and let J be the excenter opposite A. Let the angle bisector from A intersect the circumcircle at M. Then B, C, I, and J lie on a circle centered at M.
Contributed by: Jay Warendorff (March 2011)
Open content licensed under CC BY-NC-SA
See problem 2.4(a) in V. Prasolov, Problems in Plane and Solid Geometry, Vol. 1, Plane Geometry [PDF], (D. Leites, ed. and trans.).