Question

in the adjoining figure the area enclosed between the concentric circle 770 cm if the radius of the outer circle is 20 cm

Answer 1

Radius of outer circle, R = 21cm

Let the radios of inner circle = r

Area enclosed = 770 cm^2
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Area enclosed = area of outer circle - area of inner circle

$770 = \pi {R}^{2} - \pi {r}^{2} \\ \\ 770 = \pi({R}^{2} - {r}^{2}) \\ \\ 770 = \frac{22}{7} \times ( {21}^{2} - {r}^{2} )$

${21}^{2} - {r}^{2} = \frac{770 \times 7}{22} \\ \\ 441 - {r}^{2} = 245 \\ \\ {r}^{2} = 441 - 245 = 196 \\ \\ \boxed{ r = \sqrt{196} = \bold{ \red{ 14 \: cm}}}$

$\textbf{Radius of inner circle is 14 cm}$