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Question

# The area enclosed between two concentric circles is 770 cm2. If the radius of the outer circle is 21 cm, calculate the radius of the inner circle.

A
7 cm
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B
14 cm
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C
2.1 cm
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D
35 cm
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Solution

## The correct option is C 14 cmLet radius of the inner circle be r and radius of the outer circle be R=21cmArea enclosed between the two concentric circles =π(R2−r2)=770cm2⇒770=π(R2−r2)⇒770=227(212−r2)⇒770×722=441−r2⇒245=441−r2⇒r2=196⇒r=√196⇒r=14Thus, radius of the inner circle =14 cm

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