Maths-
General
Easy

Question

The area enclosed between two concentric circles is 770 cm2. If the radius of the outer circle is 21cm, calculate the radius of inner circle

hintHint:

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, r1 = 21 cm
    Let radius of inner circle = r
    Area enclosed between two concentric circles = 770 cm2.
    rightwards double arrow Area of outer circle – Area of inner circle = 770 cm2.
    straight pi (r1)2  - straight pi r2   =  770
    22 over 7 × (21)2  - 22 over 7 ×  (r)2  = 770
    22 over 7 × (212 – r2 ) = 770
    441 – r2 = 245
    r2 = 441 – 245 = 196
    r = 14 cm
    Hence, radius of inner circle is 14 cm.

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