Multiple choice

If the area of a circle is $\displaystyle 81\pi $ then its circumference is

  1. $\displaystyle 61\pi $
  2. $\displaystyle 20\pi $
  3. $\displaystyle 18\pi $
  4. $\displaystyle 16\pi $
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

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

AI explanation

Using the area formula, Area equals pi r squared, we set 81 pi equal to pi r squared and solve to find the radius is 9 cm. Next, apply the circumference formula, 2 pi r, using this radius. The circumference is 2 pi times 9, resulting in 18 pi.

Keep practicing — related questions