Half an Annulus

Can you locate the point on an annulus that splits it into two areas?
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Problem



Image
Half an Annulus


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.