Multiple choice

The area of a circle is $81\pi$. What is the circumference of the circle?

  1. $6\pi$
  2. $12\pi$
  3. $18\pi$
  4. $24\pi$
  5. $36\pi$
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C Correct answer
Explanation

Area = pi*r^2 = 81*pi, so r^2 = 81, r = 9. Circumference = 2*pi*r = 2*pi*9 = 18*pi.

AI explanation

Using the area of a circle formula, area equals pi times the square of the radius, so 81 times pi equals pi times r squared, making the radius 9. The circumference formula is 2 times pi times the radius, which becomes 2 times pi times 9. The circumference is 18 times pi.

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