The area enclosed between the concentric circles is 770 cm2. If the radius of the outer circle is 21 cm, find the radius of the inner circle.
Let the radius of the inner circle be r cm. Then,
π×21×21−π×r2=770
⇒π(441−r2)=770
⇒227×(441−r2)=770
⇒441−r2=770×722=245
⇒r2=196
⇒r=√196=14 cm
Hence, the radius of the inner circle is 14 cm