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Thread: Geometric forumlas related as integrals

  1. #1
    Junior Member
    Sep 2010

    Geometric forumlas related as integrals

    I noticed today that the formulas for the area and circumference of a circle are related as integrals. The same goes for the the volume and surface of a cube. In other words:
    Geometric forumlas related as integrals-picture-12.png
    Also, when I draw the graphs of the two formulas for the circle, they intersect when r=2. With the sphere they intersect at r=3. Does this stem from the fact that the two objects have 2 and 3 dimensions, respectively?

    I think I would understand integrals a whole lot better if I knew how this all fits together. So can anyone explain, or at least point me in the right direction?


    Edit: mods, can you correct my spelling in the title? I obviously meant formulas.
    Last edited by TwoPlusTwo; Oct 8th 2010 at 07:27 AM.
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  2. #2
    MHF Contributor
    Oct 2009
    Suppose that V(r) is the ball volume dependent on the radius. Then dV(r) = V'(r)dr is the volume of a thin sphere enveloping the ball whose thickness is dr. On the other hand, the volume of this sphere is S(r)dr where S(r) is the surface area. Therefore, S(r) = V'(r).

    Now, suppose that for an $\displaystyle n$-dimensional ball, $\displaystyle V_n(r)=ar^n$ for some constant $\displaystyle a$. Then $\displaystyle V_{n-1}(r)=V_n'(r)=anr^{n-1}$ and the solution to the equation $\displaystyle V_n(r)=V_{n-1}(r)$ is $\displaystyle r = n$.
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