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Question
Find the area of a flat circular ring formed by two concentric circles (circles with the same centre) whose radii are 9 cm and 5 cm.
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Solution
External radius, r1 = 9 cm
Internal radius, r2 = 5 cm
Area of ring = `pir_1^2 - pir_2^2`
= `pi(r_1^2 - r_2^2)`
= `pi(9^(2) - 5^(2))`
= `22/7[81 - 25]`
= `22/7 xx 56`
= 176 cm2
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