Question

Find the area of a circle whose radius is
(i) 7 cm
(ii) 2.1 m
(iii) 7 km

Answer 1

(i) We know that the area A of a circle of radius r is given by A = $\mathrm{\pi }{r}^2$.
Here, r = 7 cm
∴ $A=\frac{22}{7}\times {\left(7\right)}^2{\mathrm{cm}}^2$.
$\Rightarrow A=\left(\frac{22}{7}\times 7\times 7\right){\mathrm{cm}}^2=\left(22\times 7\right){\mathrm{cm}}^2=154{\mathrm{cm}}^2$.
(ii) ​We know that the area A of a circle of radius r is given by A = $\mathrm{\pi }{r}^2$.
Here, r = 2.1 m
∴ $A=\frac{22}{7}\times {\left(2.1\right)}^2{\mathrm{m}}^2$.
$\Rightarrow A=\left(\frac{22}{7}\times 2.1\times 2.1\right){\mathrm{m}}^2=\left(22\times 0.3\times 2.1\right){\mathrm{m}}^2=13.86{\mathrm{m}}^2$.
(iii) ​We know that the area A of a circle of radius r is given by A = $\mathrm{\pi }{r}^2$.
Here, r = 7 km
∴ $A=\frac{22}{7}\times {\left(7\right)}^2{\mathrm{km}}^2$.
$\Rightarrow A=\left(\frac{22}{7}\times 7\times 7\right){\mathrm{km}}^2=\left(22\times 7\right){\mathrm{km}}^2=154{\mathrm{km}}^2$.