Bunuel
The solid shows a truncated cone. R represents the radius of the base and r represents the radius of the truncated top. If the height (h) of the part of the cone removed from the top is 4, \(r = 5\), \(R = 10\), and \(h = 6\), what is the ratio of the volumes of the truncated cone and of the top removed?
A. 1:3
B. 1:9
C. 3:1
D. 9:1
E. 9:10
Volume of cone = \(\frac{1}{3} \pi r^2 h \)
If the height of part of the cone that is removed is 4 and the height of the truncated cone is 6, the total height of original cone is
10.
Let's call this
Big ConeTherefore Volume of the
whole cone = \(\frac{1}{3} \pi 10^2 10 \)
Height of the part of cone that is removed is 4. Lets call this
Small cone.
Therefore volume of the
Small Cone = \(\frac{1}{3} \pi 5^2 4 \)
Volume of the
Truncated cone = Volume of
Big Cone - Volume of
Small Cone= \(\frac{1}{3} \pi 10^2 10 \) - \(\frac{1}{3} \pi 5^2 4 \)
Therefore ratio of Truncated Cone to Small Cone is
\(\frac {\frac {1} {3} \pi 10^2 10 - \frac{1}{3} \pi 5^2 4}{\frac{1}{3} \pi 5^2 4}\)
(\(\frac {1}{3} \pi \) gets cancelled.)
=\(\frac {100(10)-100}{100}\\
= \frac {10-1}{1}\\
=\frac {9}{1}\)
Therefore ratio = 9:1
Answer = D