Stage: 4 Challenge Level: 
Two intersecting circles have a common chord AB. The point C moves on the circumference of the circle C1. The straight lines CA and CB are extended to meet the circle C2 at E and F respectively.
As the point C moves what do you notice about the chord EF?
Give a proof of your conjecture.
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Chords and segments. Mathematical reasoning & proof. Games. Real world. Working systematically. Scale factors. Visualising. Dynamic geometry. Circle theorems. Similarity.
Published September 1998.
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