What Property is this? a.b = b.a
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I think you mean circumference...Area is 
. We have the area so we need to use it to solve for the radius which we will then use in the circumference formula. 
. Divide both sides by pi to get 
. Of course the simplification of the left side gives us 
and r = 4. Now fill that in to the circumference formula, which is 
, to get C
which is a circumference of 
