Question

Find the area of a flat circular ring formed by two concentric circles (circles with same centre) whose radii are 9 cm and 5 cm.

Answer 1

External radius, $r_1=9cm$

Internal radius, $r_2=5cm$

$\therefore$ Area of ring $=\pi r_1^2-\pi r_2^2$

$=\pi (r_1^2-r_2^2)$

$=\pi (9^2-5^2)$

$=\frac{22}{7}[81-25]$

$=\frac{22}{7}\times 56$

$=176c{m}^2$