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General

Easy

Question

# The area enclosed between two concentric circles is 770 cm^{2}. If the radius of the outer circle is 21cm, calculate the radius of inner circle

Hint:

### Area enclosed between two concentric circles is the difference of the areas of two circles

## The correct answer is: 14 cm

### It is given that radius of outer circle, r_{1} = 21 cm

Let radius of inner circle = r

Area enclosed between two concentric circles = 770 cm^{2}.

Area of outer circle – Area of inner circle = 770 cm^{2}.

(r_{1})^{2} - r^{2 } = 770

× (21)^{2} - × (r)^{2} = 770

× (21^{2} – r^{2 }) = 770

441 – r^{2} = 245

r^{2} = 441 – 245 = 196

r = 14 cm

Hence, radius of inner circle is 14 cm.

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