# A hollow sphere of internal and external diameters $4 \mathrm{~cm}$ and $8 \mathrm{~cm}$ respectively is melted into a cone of base diameter $8 \mathrm{~cm}$. Calculate the height of the cone.

Given:

A hollow sphere of internal and external diameters $4 \mathrm{~cm}$ and $8 \mathrm{~cm}$ respectively is melted into a cone of base diameter $8 \mathrm{~cm}$.

To do:

We have to find the height of the cone.

Solution:

Outer diameter of hollow sphere $=8 \mathrm{~cm}$

Inner diameter of hollow sphere $=4 \mathrm{~cm}$

This implies,

Outer radius $R=\frac{8}{2}$

$=4 \mathrm{~cm}$

Inner radius $r=\frac{4}{2}$

$=2 \mathrm{~cm}$

Volume of the hollow sphere $=\frac{4}{3} \pi(\mathrm{R}^{3}-r^{3})$

$=\frac{4}{3} \pi[4^{3}-2^{3}]$

$=\frac{4}{3} \pi[64-8]$

$=\frac{4 \pi}{3} \times 56$

$=\frac{224}{3} \pi \mathrm{cm}^{3}$

Diameter of the base of the solid cone $=8 \mathrm{~cm}$

This implies,

Radius of the base of the solid cone $r_1=\frac{8}{2}$

$=4 \mathrm{~cm}$

Let the height of the cone be $h$.

Therefore,

$\frac{1}{3} \pi r_{1}^{2} h=\frac{224}{3} \pi$

$\Rightarrow \frac{1}{3} \pi(4)^{2} h=\frac{224}{3} \pi$

$\Rightarrow \frac{16}{3} \pi h=\frac{224}{3} \pi$

$\Rightarrow h=\frac{224 \pi \times 3}{3 \times 16 \pi}$

$\Rightarrow h=14\ cm$

The height of the cone is $14 \mathrm{~cm}$.

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Updated on: 10-Oct-2022

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