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

Answer: 418

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
$=3\pi R^2+\pi r^2=\pi \left(3\times 6^2+5^2\right)$
$=\pi (108+25)=418\pi {\mathrm{cm}}^2$