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