The circumference of a circle whose area is 81πr², is
(a) 9πr
(b) 18πr
(c) 3πr
(d) 81πr

Solution:

Given, area of circle = 81πr²

We have to find the circumference of the circle

Let the radius of the circle be R

Area of circle = πr²

Where, r is the radius of the circle

81πr² = πR²

R² = 81r²

Taking square root,

R = 9 r

Circumference of circle = 2πr

Where, r is the radius of the circle

= 2πR

= 2π(9r)

= 18πr

Therefore, the circumference is 18πr

✦ Try This: The circumference of a circle whose area is 100πr², is

☛ Also Check: NCERT Solutions for Class 7 Maths Chapter 11

NCERT Exemplar Class 7 Maths Chapter 9 Problem 34

The circumference of a circle whose area is 81πr², is (a) 9πr, (b) 18πr, (c) 3πr, (d) 81πr

Summary:

The circumference of a circle whose area is 81πr² is 18πr

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