Stage: 4 Challenge Level: 
A white cross is placed in a red disc as shown and the resulting
figure is symmetrical.
What is the area of the disc still showing?
Right angled triangles. Sine rule. Circles. Area. Rotations. Long problems. Measures - generally. Complex numbers. Reflections. Pythagoras' theorem.
Published December 2001,January 2002.
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