Find the radius of the inner circle, Mathematics

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The area enclosed between two concentric circles is 770cm2. If the radius of the outer circle is 21cm, find the radius of the inner circle. (Ans :14cm)

Ans: Π R2 - Π r2 = 770
Π (212 - r2) = 770

212 - r2 = 770/22 x 7 = 70/2 x 7

r2 =441 -490/2= 441 - 245=196

r= ± 14
r=14cm


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