Stage: 3 and 4 Challenge Level: 
Think about the radii.
Making and proving conjectures. Dynamic geometry. Visualising. Stage 5 - Reviewed 2012. Number theory. Circles. Generalising. Inequality/inequalities. Mathematical reasoning & proof. Manipulating algebraic expressions/formulae.
Published May 1999,August 2008.
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email: nrich@damtp.cam.ac.uk