The proof of the volume of the truncated cone

1. Apr 30, 2008

Jasty

Please I need a respectable proof how to get the volume of the truncated cone. I need it really quick. So please could you help me. No numbers just "the method" how to get that formula. Thanks.

2. Apr 30, 2008

HallsofIvy

Staff Emeritus
Starting from where? If you can use the formula for volume of a cone, its trivial.

3. Apr 30, 2008

mathman

If the truncation is parallel to the base, Halls of Ivy is correct. Otherwise it is some what more complicated.

4. Apr 30, 2008

HallsofIvy

Staff Emeritus
Yeah, I'll leave that to mathman!

5. May 1, 2008

Jasty

I mean the most basic conditions. I can imagine the whole cone and than remove the top. Is there any other way how to prove it?

6. May 1, 2008

HallsofIvy

Staff Emeritus
Let C be a truncated, right circular cone with height H, upper radius R1 and lower radius R2. Set it up on a coordinate system with the center of the base at (0,0,0), and center of the top at (0,0,H). Looking at it from the side, so that you see the xz-plane, you see a "trapezoid" with one side starting at (R2,0,0) and ending at (R1,0,H). Since any nonvertical line in the xz-plane can be written in the form z= Ax+ B. you must have 0= R2A+ B and H= R1A+ B. Subtracting the first from the second, H= (R1-R2)A so A= H/(R1-R2), B= -R2A so B= -R2H/(R1-R2). The equation of the line is z= H(x-R2)/(R1-R2) or you can write it x= (R1-R2)z/H+ R2.

Now imagine the entire cone, divided into thin disks: each has thickness "dz" and radius, x= (R1-R2)z/H+ R2 so area $\pi [(R1-R2)z/H+ R2)^2$ and volume $\pi [(R1-R2)z/H+ R2)^2dx$. To find the entire volume integrate that from z= 0 to z= H.

Last edited: Feb 18, 2010
7. May 4, 2008

Jasty

ok thanks all.

8. May 4, 2008

trambolin

Can we do this by using a deformation matrix and then using the regular formula if it is not parallel to the base? Just out of curiosity, couldn't see it right away

9. Feb 18, 2010