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# Half an Annulus

##### Age 14 to 16 Short Challenge Level: The shaded region in the diagram, bounded by two concentric circles (circles whose centres are at the same point), is called an annulus.

The circles have radii 2 cm and 14 cm.

The dashed circle has the same centre as the other two and divides the area of this annulus into two equal areas.

What is its radius?

This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.