Math: Geometry Problem 1006:
Tangent Circles, Common External Tangent, Bicentric
Trapezoid, Incenter, Circumcenter. Level: School, College.
Circles O and Q are tangent externally
at T (see the figure below). If AB and CD are common external
tangents, prove that ABCD is a bicentric trapezoid with incenter T and circumcenter O1.