fmcgee1
Padfoot
yashikaaggarwal
A hollow right circular cylinder of radius r and height 4r is standing vertically on a plane. If a solid right circular cone of radius 2r and height 6r is placed with its vertex down in the cylinder, then the volume of the portion of the cone outside the cylinder is:
A. 8/(3πr^3)
B. 2πr^3
C. 9/(8πr^3)
D. 7πr^3
E. 4/(3πr^3)
please see the image attached for solution.
trick here is: max radius of cone that can be put inside hallow cylinder is "r"
and corresponding height of cone inside the cylinder is "3r"
Can't read half of the writing on this. Is that a 6r-x, or is that a pi symbol?
Apology for the inconvenience!
Given: Radius of cylinder = r
Max radius a cone of any radius that can be accommodated by this hollow cylinder of radius "r" = r
therefore, Radius of Cone accommodated inside the cylinder = r
now, we need to find what height of cone is accommodated inside the cylinder, given that the total height of cone = "6r"
from the figure attached,
we can see that
Triangle PQS is similar to triangle PRT (by AAA similarity)
\((Angle Q= Angle R =90 degree)\)
\((Angle P = Angle P)\)
since, 2 angle of a triangle are equal to 2 corresponding angle of another triangle, their 3rd angle will be equal too!
\(Angle PSQ = Angle PTR\)
since, Triangle PQS is similar to Triangle PRT
we have the below:
\(PQ/PR = QS/RT\) -------- (1)
\(QS= r,\) \(RT= 2r,\) \(PR= 6r\)
PQ = max height accommodated inside the cylinder,
A word of caution here: max height of cone accommodated inside cylinder is not equal to max height of cylinder "4r"
Lets say height of cone remaining outside the cylinder be "x"
height of cone, therefore, accommodated inside the cylinder = total height of cone - height of cone remaining outside the cylinder
=>\( 6r-x = PQ\)
Substituting value of QS, RT, PR and PQ we get:
\((6r-x)/6r = (r/2r)\)
or \(x= 3r\), therefore, \(6r-x = PQ = 3r\).
Volume of whole Cone = \((1/3)*pi*r^2*h = (1/3)*pi*2r^2*6r = 8*pi*r^3\)
Volume of Cone inside Cylinder = \((1/3)*pi*r^2*h = (1/3)*pi*r^2*3r = pi*r^3\)
So, Voulme of Cone outside Cylinder = \(8*pi*r^3 - pi*r^3 = 7*pi*r^3\)
Attachments

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