Question

The outer radius and the inner radius of a hollow hemisphere are $6 \mathrm{~cm}$ and $5 \mathrm{~cm}$, respectively. The total surface area (in $\mathrm{cm}^2$ ) of the hollow hemisphere is____

Answer 1

Let $R$ and $r$ be the outer and inner radius of the hollow sphere.
$\Rightarrow R=6 \text { and } r=5$
The total surface area of the hemisphere
$ =3 \pi R^2+\pi r^2=\pi\left(3 \times 6^2+5^2\right) $
$ =\pi(108+25)=418 \pi \mathrm{cm}^2$