The circles O and O_{1} intersect at A and B (see the figure above),
CD is the common external tangent, CA intersects circle O_{1} at E,
DA intersects circle O at F. If FC and
ED intersect at G, prove that angles ABC and DBG are congruent.
Dynamic Geometry Environment (DGE) or Interactive Geometry Software
(IGS) of Problem 900
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Solution/Comment  by Antonio Gutierrez
