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In every circle . . .
Area = (pi) x (radius)2
Diameter = 2 x radius
Circumference = (pi) x diameter
In this circle, Area = (pi) R2 = 81
R2 = 81/pi
R = sqrt( 81/pi )
Diameter = 2 x sqrt( 81/pi )
Circumference = (pi) x 2 x sqrt( 81/pi ) = 2 x sqrt( 81 pi ) = 18 sqrt(pi) = 31.9042 (rounded)
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