


Online Geometry Problem 661: Intersecting Circles,
Centers, Secant, Parallelogram. Level: High School,
Honors Geometry, College, Mathematics Education

The figure shows the intersecting circles O
and O' at A and B. O'O extended and circle O meet at C. D is a point on
circle O', DA and DB meet circle O at E and F, respectively. Prove that
DECF is a parallelogram.






