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Homework Help: Integration of partial derivatives

  1. Aug 5, 2012 #1
    1. The problem statement, all variables and given/known data

    The problem is attached in the picture. The top part shows what is written in the book, but im not sure how they got to (∂I/∂v)...


    3. The attempt at a solution

    It's pretty obvious in the final term that the integral is with respect to 't' while the differential is with respect to 'v' . How did they simply convert F(x,v) into f(x,v)?
    1. The problem statement, all variables and given/known data
     

    Attached Files:

  2. jcsd
  3. Aug 5, 2012 #2

    HallsofIvy

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    You understand that the variable of integration is a "dummy" variable don't you? That the [itex]\int_a^x f(t)dt[/itex] is a function of x, not t.

    Here, an example would be
    [tex]\int_{t= 3x}^{x^2} t^2- 2x dt= \left[\frac{1}{3}t^3- 2xt\right]_{3x}^{x^2}= \frac{1}{3}x^6- 2x^4- \left(\frac{1}{3}(27x^3)- 6x^2\right)[/tex]
    a function of x, not t.
     
  4. Aug 5, 2012 #3
    Yes, but how can you reverse the integration by ∂/∂v ? Shouldn't it be ∂/∂t instead?

    It's like saying F(x,y) = int f(x,y) dy

    then

    f(x,y) = ∂/∂z F(x,y)


    when they are clearly different variables - z and y.
     
  5. Aug 5, 2012 #4

    HallsofIvy

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    If you say "yes" then you are you saying that you understand that this integral is NOT a function of t so it cannot be differentiated with respect to t. Go back and read what I said again. [itex]\int_u^v f(x,t)dt[/itex] is a function of u, v, and x, NOT t.

    You are the one who is try to differentiate with an incorrect variable.
     
  6. Aug 5, 2012 #5
    So we simply look at what's the end-product, F(x,v) instead of the intermediate step?
     
  7. Aug 5, 2012 #6
    Then does this hold?

    I = F(x,v) - F(x,u)

    ∂I/∂x = f(x,v) - f(x,u)
     
  8. Aug 5, 2012 #7

    HallsofIvy

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    When they say [itex]I= \int_{u(x)}^{v(x)}f(x,t)dt[/itex] and then [itex]I= F(x,v)- F(x,u)[/itex] they are really saying that [itex]F(x,v)= \int_a^{v(x) f(x,t)dt[/tex] and [itex]F(x,u)= \int_u^a f(x,t)dt[/tex] where a is any constant.

    By the fundamental theorem of Calculus,
    [tex]\frac{\partial}{\partial v}F(x, v)= f(x,v)[/tex]

    We can write [itex]F(x,u)= \int_u^a f(x,t)dt= -\int_a^u f(x,t)dt[/itex] so that
    [tex]\frac{\partial}{\partial u}F(x,u)= -f(x,u)[/itex]

    Now, to find the derivative with respect to x use the chain rule.
     
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