Question

Find the circumference of a circle whose diameter is
(i) 7 cm
(ii) 4.2 cm
(iii) 11.2 km

Answer 1

(i) We have:
Diameter = 7 cm
∴ Radius = $\frac{7}{2}$ cm
Let C be the circumference of a circle. Then,
C = 2$\mathrm{\pi }$r
∴ C = $2\times \frac{22}{7}\times \frac{7}{2}=22\mathrm{cm}$.
(ii) We have:
Diameter = 4.2 cm
∴ Radius = $\frac{4.2}{2}\mathrm{cm}=2.1\mathrm{cm}$
Let C be the circumference of a circle. Then,
C = 2$\mathrm{\pi }$r
∴ C = $2\times \frac{22}{7}\times 2.1=13.2\mathrm{cm}$.
(iii) ​​We have:
Diameter = 11.2 km
∴ Radius = $\frac{11.2}{2}\mathrm{km}=5.6\mathrm{km}$
Let C be the circumference of a circle. Then,
C = 2$\mathrm{\pi }$r
∴ C = $2\times \frac{22}{7}\times 5.6=35.2\mathrm{km}$.