Given: Let the radius of the inner circle = r
Area enclosed between two concentric circles = 770cm2
and Radius of the outer circle, R = 21cm
∴ The area enclosed between two concentric circles
= Area of the Outer circle – Area of the inner circle
770 = πR2 – πr2
770 = π(212 – r2)
245 = 441 – r2
⇒ r2 = 441 – 245
⇒ r2 = 196
⇒ r = √196
⇒ r = ±14
⇒ r = 14cm [taking positive square root, because radius can’t be negative]
Hence, the radius of the inner circle is 14cm.