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Old 06-16-2008   #2
Sillysidley

 
Join Date: Oct 2006
Posts: 822
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Well, let the radius of the smaller circle be r, so the radius of the larger circle is 2r-1. We then have , since we just divided out the pi. Now, I just eyed it out and found r=4, so our answer is 7. If you can't see that, you can expand to get , so we know r must be 4.
Or, after getting , you can apply difference of squares to get and you can either make it a quadratic again, or in this case it's easy to examine factors and find r=4.
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