Stage: 5 Challenge Level: 
A circle with radius 1 and a circle with radius 2 touch at a point. A third circle fits between these two circles so that all three touch each other and all three have a common tangent. What is the radius of the smallest circle?
Summation of series. Pythagoras' theorem. Surds. epsilons. Tangents. Circles. Stage 5 - Reviewed 2012. alphas. Cartesian equations of circles. Mathematical reasoning & proof.
Published February 1999.
Copyright © 2007. University of Cambridge. All rights reserved.
NRICH is part of the family of activities in the Millennium Mathematics Project, which also includes the Plus and Motivate sites.
email: nrich@damtp.cam.ac.uk