Question

Question 34

The circumference of a circle whose area is $81\pi r^2$, is

(a) $9\pi r$
(b) $18\pi r$
(c) $3\pi r$
(d) $81\pi r$

Answer 1

Let the radius of circle be R.
$\therefore$ Area of circle $=\pi R^2$

We have area of circle $=$ $81\pi r^2$

$\Rightarrow 81\pi r^2=\pi R^2$

$\Rightarrow R=\sqrt{81r^2}$
$\Rightarrow R=9r$
Now, circumference of circle $=2\pi R=2\pi \left(9r\right)=18\pi r$

Hence, Option (b) is correct.