Question

in adjoining figure, the area enclosed between the concentric circles is 770cm². if the radius if the outer circle is 21cm, calculate the radius if the outer circle.

Answer 1

Area between the concentric circle = area of bigger circle - area of smaller circle
770 = π r^2 - πr^2
770 = 22 / 7 × 21 ×21 - 22 / 7 × r^2
770 - 1386 = -22 / 7 × r^2
-610×7÷[- 22] = r^2
194.090 = r^2
14cm = r