Question

In the adjoining figure The area enclosed between the concentric circles is 770cmsquare . If the radius of the outer circle in 21 cm calculate the radius of the inner circle

Answer 1

Area =  \pi  (R²-r²)
770 = 22/7 × [(21)² - r²]
770×7÷22 = 441-r²
441 - 245 = r²
r² = 196
r = √196
r = 14cm

Answer 2

Answer:

14 cm.

Step-by-step explanation:

Let the Radius of inner circle be r

Outer radius R = 21 cm

The area enclosed between the concentric circles is 770 cm square .

Formula of area between concentric circle = Area of outer circle - Area of inner circle

Area between concentric circle = $\pi R^2-\pi r^2$

So, $770=\pi (21^2-r^2)$

$770=\frac{22}{7}\times(441-r^2)$

$770\times \frac{7}{22}=441-r^2$

$70\times \frac{7}{2}=441-r^2$

$35\times 7=441-r^2$

$245=441-r^2$

$r^2=441-245$

$r^2=196$

$r=14$

Hence the inner radius is 14 cm.