Question 1141653
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See the lesson

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Surface-area/Area-of-a-regular-n-sided-polygon-via-the-radius-of-the-circumscribed-circle.lesson>Area of a regular n-sided polygon via the radius of the circumscribed circle</A> &nbsp;and 

in this site.