Find the surface area of a sphere of diameter 21 cm.

Given,

Diameter of sphere (d) = 21 cm

Radius of sphere (r) = $\dfrac{\text{Diameter}}{2} = \dfrac{21}{2}$ = 10.5 cm

By formula,

Surface area of a sphere (A) = 4πr²

$A = 4 \times \dfrac{22}{7} \times (10.5)^2 \\[1em] = 4 \times \dfrac{22}{7} \times 10.5 \times 10.5 \\[1em] = 4 \times 22 \times 1.5 \times 10.5 \\[1em] = 1386 \text{ cm}^2.$

Hence, the surface area of a sphere = 1386 cm².