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प्रश्न
The circumference of a circle whose area is 81 πr2, is ______.
विकल्प
9 πr
18 πr
3 πr
81 πr
उत्तर
The circumference of a circle whose area is 81 πr2, is 18 πr.
Explanation:
Let the radius of circle be R
∴ Area of circle = πR2
⇒ 81πr2 = πR2
⇒ R = `sqrt(81r^2)`
⇒ R = 9r
Now, circumference = 2πR = 2π(9r) = 18πr
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