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The area enclosed between two concentric circles is 770 sq cm. If the radius of outer circle is 21cm. Find the radius of the inner circle.

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Area of a circle = πr^{2}

where r is the radius

The area between the two circles is given as 770 sqcm.

If the radius of the outer circle is 21cm, it's area will be:

π(21)^{2} = 441π

Therefore, the area of the inner circle = 441π - 770

= 615.44 sqcm

Also, area = πr^{2}

615.44 = πr^{2}

divide both sides by π

r^{2} = 195.9

r = 14cm

Therefore, the radius of the inner circle is 14cm

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