If the area of a circle is 81pi square feet, find its circumference.  Choose the correct answer.
A) 8  B) 28  C) 18  D) 48  E) 38
The area is given by pi × r × r. Hence   pi × r × r = 81 pi   r × r = 81 ; hence r = 81 feet   The circumference is given by   2 × pi × r = 2 × pi × 9 = 18 pi feet   correct answer 18
<result>C</result>
If the area of a circle is 81pi square feet, find its circumference.