# The volume of a cube and a cylinder.

I was wondering if the formulas for the volume of, for instance, a cube and a cylinder are definitions or if they can be proved. Does anybody know :)?

Hootenanny
Staff Emeritus
Gold Member
I was wondering if the formulas for the volume of, for instance, a cube and a cylinder are definitions or if they can be proved. Does anybody know :)?
One can derive the volumes of various 3D shapes using volume integrals with appropriate limits.

The proof for the formula to derive volumes of those various 3D shapes is based on the volume of a cylinder, atleast the one I learned..

The volume of a cube based on the volume of cylinder?? Are you sure it wasn't the other way round?

Anyway, as Hottenanny pointed out you can just calculate these kind of volume using integrals or (essentially the same) http://fr.wikipedia.org/wiki/Principe_de_Cavalieri" [Broken] (sorry, only available in French and German )

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For simplicity consider a unit square.
We define the area of a unit square as 1. (at least I think we do!!)
Seems to say so here:-
http://mathforum.org/library/drmath/view/60392.html

I think however we could have defined the area of a unit square a 7 or 12.738 or 1/4
or even -0.0009300203.
It would just make the maths a bit harder it we did!

So it seems to me it is a definition so proving it is trivial, for example an exam question
might be:-

a) Given the area of a unit square is 1, show the area of a unit square is 1.

I don't think that will be worth too many marks!!

Going on to volume, I think we define a unit volune as one, but it could have been
defined as any number, one just makes the numbers easier to work with.

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HallsofIvy
Homework Helper
Yes, a volume of "1" is defined as the volume of a "unit" cube- that is, the volume of a cube of length "1" on each side.

The fact that the volume of an n by n cube, for n an integer, is n3 does require a proof and the fact that the volume of an x by x cube, with x any real number, is x3 requires a significantly harder proof.

Once we are "given" the area of a circle, the proof that the volume of a right circular cylinder is $\pi r^2 h$ is fairly simple.

Would you mind elaborating on "x by x cubes" where x is not an integer ...? Or do you know a link/book where I could read about such things?
Thanks.