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Integration Coneptual Question

  1. Jul 3, 2006 #1
    What's the difference between dx times dy and a point? Having trouble thinking about this... it's been hurting my head, any help would be greatly appreciated.
  2. jcsd
  3. Jul 3, 2006 #2


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    What is the context of the question? "dx times dy" might be the "differential of area". It is, in any case, a differential which doesn't have anything to do with a point.
    The question seems to me to be a lot like asking "What is the difference between the number 5 and a point?":confused:
  4. Jul 3, 2006 #3
    No, I mean an infinitely small change in the x direction times an infinitely small change in the y direction.
  5. Jul 3, 2006 #4
    it would be an infinitely small change in the area enclosed by a curve(s) in a plane. a point is just a point.
  6. Jul 3, 2006 #5


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    A point is a geometrical object. dx dy is an algebraic object.
  7. Jul 4, 2006 #6
    Hmm, could someone define a point for me... i think the problem is that I define a point as an indivisible amount of space, which is probably wrong, any help?
  8. Jul 4, 2006 #7


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    A point is an element of a particular vector space (or, roughly, number system), for example R2
  9. Jul 4, 2006 #8
    A point is an abstraction invented by Euclid.

    dx*dy cannot be "visualized" as if it were a geometric object. Instead, visualize what one does with it, in context. Doing 2d integrals? Visualize finite partitions of a region (perhaps a grid of widths ΔxΔy), their Riemann sums under a function f, and the limiting behavior of all that. dx*dy is an informal way of saying we're looking at some limiting behavior, of finite Δx*Δy. Infinitesimals don't exist on their own - they exist with reference to some limiting process we're describing. Thus dx*dy has more mathematical meaning then just the 'point' at which it is located.
  10. Jul 4, 2006 #9
    I see my problem, I was trying to apply my world to mathematics too readily, thanks for the help everyone.
  11. Jul 8, 2006 #10
    I guess a point can be considered a geometric shape with n sides and area, perimeter and volume =0 like a triangle with one null angle...
  12. Jul 8, 2006 #11
    LeBesgue's definition of a "point" is, as far as I am concerned, the best
    conceptualisation of this abstract object. The downside is that it requires a
    certain mathematical background. He stills manages to make the difference between
    the differential area dx*dy and a point quite clear though.
    Last edited: Jul 8, 2006
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