Theory:

Hemisphere:
A section of the sphere cut by a plane through any of its great circles is a hemisphere. In another way, we can say, one half of a sphere is called a hemisphere.
Volume of a hemisphere:
Let \(r\) be the radius of a sphere.
 
Volume of a hemisphere \(=\) 12 \(\times\) Volume of a sphere
 
\(=\) 12×43πr3
 
\(=\) 23πr3
 
Volume of a hemisphere \(=\) 23πr3 cu. units
 
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Volume of hollow hemisphere (volume of the material used):
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Let \(r\) be the inner radius and \(R\) be the outer radius of the hollow hemisphere.
 
Volume of hollow hemisphere \(=\) Volume of the outer hemisphere \(-\) Volume of the inner hemisphere
 
\(=\) 23πR3 \(-\) 23πr3
 
\(=\) 23πR3r3
Volume of a hollow hemisphere \(=\) 23πR3r3 cu. units
Important!
The value of \(\pi\) should be taken as 227 unless its value is shared in the problem.