Stage: 5 Challenge Level: 
The proof depends on identifying two sets of similar triangles and spotting that they are arranged around the centre of the inner circle in such a way that they can be used to show that they fit together exactly as the angles add up to 360 degrees. The method is elementary but calls for a systematic approach.
Mathematical reasoning & proof. Radius (radii) & diameters. Games. Interactivities. Angles at a point/on a line. Making and proving conjectures. Circle theorems. Scale factors. Similarity. Dynamic geometry.
Published February 2004.
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