Question

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

• A. $6\pi$
• B. $12\pi$
• C. $18\pi$
• D. $24\pi$
• E. $36\pi$

Answer 1

Answer: (C)

$18\pi$

Given area of a circle is $81\pi$.
We know formula for area of a circle is $\pi r^2$
So,$\pi r^2=81\pi$
$\Rightarrow r^2=81$
$\Rightarrow r=9$
Formula for circumference is $2\pi r$.
So putting $r=9$ circumference will be
$2\pi 9=18\pi$