Source: rec.puzzles, Eva Zerz in July 1995,
Dennis Yelle in February, 1996.
A, B and C are in a straight line.
Perpendicular lines CA and CB are both tangent to circle O with
AB passes through O, the center of the circle.
AO intersects the circle at D.
BO intersects the circle at E.
If BE=1, find AD.
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