A golf ball has diameter equal to 4.2 cm.
Question:

A golf ball has diameter equal to 4.2 cm. Its surface has 200 dimples each of radius 2 mm. Calculate the total surface area which is exposed to the surroundings assuming that the dimples are hemispherical.

Solution:

Surface area of ball

$=4 \pi r^{2}$

$=4 \pi\left(\frac{4.2}{2}\right)^{2}$

$=17.64 \pi \mathrm{cm}^{2}$

Total surface area exposed

$=\mathrm{SA}$ of ball $-200\left(\pi \mathrm{r}^{2}-\frac{4 \pi r^{2}}{2}\right)$

$=17.64 \pi-200 \pi r^{2}$

$=17.64 \pi-8 \pi$

$=80.5 \mathrm{~cm}^{2}$