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Calculating dy

  1. Aug 28, 2011 #1
    1. The problem statement, all variables and given/known data
    I was messing around online when I found this: [itex]\frac{dy}{2}[/itex] = 2x. This was derived from the function y = x2. I had never really seen anything like this before. When I solved for "dy," I got 4x. However, for example, when x changes from 0 to 2, the y changes from 0 to 4. Interestingly enough, "dy" is represented as being 4x and not just 4. Can someone point out what I did wrong?


    2. Relevant equations
    y = x2
    [itex]\frac{dy}{2}[/itex] = 2x


    3. The attempt at a solution
    Pretty much explained in the "problem statement" by accident.
     
  2. jcsd
  3. Aug 28, 2011 #2

    SteamKing

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    You find a lot of things on the internet. Shockingly, not all of them are true.
     
  4. Aug 28, 2011 #3
    That is true without a doubt! lol. How would I go about solving something like this though? I am given delta x and the derivative and have to find delta y. It all seems very interesting.
     
  5. Aug 28, 2011 #4

    HallsofIvy

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    You wouldn't. The equation is meaningless. You cannot have a "dy" without a corresponding "dx". In terms of "non-standard analysis", we would say that the left side of the equation is an infinitesmal while the right side is not. That can't happen.

    Since you say "This was derived from the function [itex]y = x^2[/itex]" I suspect that it was supposed to be
    [tex]\frac{dy}{dx}= x^2[/tex]
     
  6. Aug 28, 2011 #5

    Mark44

    Staff: Mentor

    The equation above should be dy = 2x dx
     
  7. Aug 31, 2011 #6
    Here the point is, in my personal opinion

    dy = 4*x

    x is a variable but as the problem ( lamely presented ) offers

    dy=0

    and

    thus
    x=4*0

    x=0 alwyas no matter what

    Either wrongly presented or gives us the line equation

    x = 0
     
  8. Aug 31, 2011 #7
    Thank you
     
  9. Aug 31, 2011 #8
    You are quite Welcome :)
     
  10. Aug 31, 2011 #9

    Mark44

    Staff: Mentor

    As HallsOfIvy already pointed out, this equation is meaningless, so there is not much point in analyzing it further.
     
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