If the triangle $ \mathrm{ABC} $ in the Question 7 above is revolved about the side $ 5 \mathrm{~cm} $, then find the volume of the solid so obtained. Find also the ratio of the volumes of the two solids obtained in Questions 7 and 8 .
A right triangle \( \mathrm{ABC} \) with sides \( 5 \mathrm{~cm}, 12 \mathrm{~cm} \) and \( 13 \mathrm{~cm} \) is revolved about the side \( 5 \mathrm{~cm} \).
To do:
We have to find the ratio of the volumes of the two solids obtained.
Solution:
Let in a triangle $ABC$,
$AB=13\ cm$
$BC=5\ cm$
$CA=12\ cm$
On revolving the right angled triangle $ABC$ about the side $BC$, we get a cone as shown in the below figure.
Therefore,
Height of the cone $h=5\ cm$
Radius of the cone $r=12\ cm$
We know that,
Volume of a cone of radius $r$ and height $h$ is $\frac{1}{3} \pi r^2h$
This implies,
Volume of the cone $=\frac{1}{3} \times \pi (12)^2 \times 5$
$=240\pi\ cm^3$
The volume of the cone so formed is $240\pi\ cm^3$.
The ratio of the volumes of the two solids so obtained $=100\pi\ cm^3:240\pi\ cm^3$
$=100:240$
$=5:12$
The ratio of the volumes of the two solids so obtained is $5:12$.