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A cubical block of side 7 cm is surmounted by a hemisphere. What is the greatest diameter the hemisphere can have? Find the surface area of the solid.
Given:
A cubical block of side $7\ cm$ is surmounted by a hemisphere.
To do:
We have to find the greatest diameter the hemisphere can have and the surface area of the solid.
Solution:
Side of the cubical block, $a = 7\ cm$
Longest diagonal of the cubical block $= a\sqrt{2}\ cm=7\sqrt{2}\ cm$
Since the cube is surmounted by a hemisphere,
Therefore the side of the cube should be equal to the diameter of the hemisphere.
Diameter of the sphere$= 7\ cm$
Radius of the sphere, $r = \frac{7}{2}\ cm$
Total surface area of the solid $=$Total surface area of the cube$-$ Inner crossâsection area of the hemisphere $+$ Curved surface area of the hemisphere
$=6a^{2} -\pi r^{2} +2\pi r^{2}$
$=6a^{2} +\pi r^{2}$
$=6\times 7\times 7+\frac{22}{7}\times \frac{7}{2} \times \frac{7}{2}$
$=294+\frac{77}{2}$
$=332.5\ cm^{2}$.