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.
Mathematical reasoning & proof. Generalising. Area. Pythagoras' theorem. Number - generally. Visualising. Circles. Combinatorics. Creating expressions/formulae. Patterned numbers.
Published April 1999.
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