SOLUTION: the area of a circular ring is 90.If the radius of the outter circle forming the ring is 10, what is the radius of the inner ring?!

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Question 397637: the area of a circular ring is 90.If the radius of the outter circle forming the ring is 10, what is the radius of the inner ring?!
Answer by robertb(5830) About Me  (Show Source):
You can put this solution on YOUR website!
Let r = radius of inner ring.
Then 10%5E2pi+-+r%5E2pi+=+90
==> 100+-+r%5E2+=+90%2Fpi
==> r%5E2+=+100+-+90%2Fpi
==> r+=+sqrt%28100+-+90%2Fpi%29+=+8.447, to 3 decimal places.