Question

# Find the total surface area of a hollow hemisphere whose outer and inner radii are 4.2cm and 2.1cm respectively.

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Hint: It is just a formula application. Remember the formula for Total Surface area of Hollow Hemisphere & you’ll get the answer.

Total Surface area of Hollow Hemisphere is, $4\pi \left( {{r_1}^2 - {r_2}^2} \right)$

Where ${r_1}$ is outer radii and ${r_2}$ is inner radii.

So, according to question

${r_1}$ = 4.2cm

${r_2}$ = 2.1cm

$\Rightarrow {\text{Area }}\left( {\text{A}} \right) = 4\pi \left( {{{4.2}^2} - {{2.1}^2}} \right)$

$\Rightarrow {\text{A = 4}}\pi \left( {4.2 + 2.1} \right)\left( {4.2 - 2.1} \right)$

$\Rightarrow {\text{A = 4}} \times \dfrac{{22}}{7} \times 6.3 \times 2.1$

$\Rightarrow {\text{A = 166}}{\text{.32}}\;{\text{sq}}{\text{.cm}}$