Stage: 3 Challenge Level: 
Prove that the area of the annulus is ${1\over 4}\pi AB^2$ where
$AB$ is a tangent to the inner circle.
Number - generally. Visualising. Pythagoras' theorem. Mathematical reasoning & proof. Networks/Graph Theory. Algebra - generally. Creating expressions/formulae. Area. Generalising. Circles.
Published April 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
