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Homework Help: Chain rule problem with partial derivatives

  1. Feb 20, 2008 #1
    [SOLVED] Chain rule problem with partial derivatives

    1. The problem statement, all variables and given/known data
    Suppose that z = f(u) and u = g(x,y). Show that..

    [tex]\frac{\partial^{2} z}{\partial x^{2}}[/tex] = [tex]\frac{dz}{du}[/tex] [tex]\frac{\partial^{2} u}{\partial x^{2}}[/tex] + [tex]\frac{d^{2} z}{du^{2}}[/tex] [tex]\frac{(\partial u)^{2}}{(\partial x)^{2}}[/tex]

    2. Relevant equations
    [tex]\frac{\partial z}{\partial x}[/tex] = [tex]\frac{dz}{du}[/tex] [tex]\frac{\partial u}{\partial x}[/tex]

    based on the chain rule
    3. The attempt at a solution
    Based on the first order partial derivative above, I would think that using the product rule we can find the second order partial dervative of z w.r.t. x

    Using my intuition, I consider [tex]\frac{dz}{du}[/tex] and [tex]\frac{\partial u}{\partial x}[/tex] like different terms and then apply the product rule.

    However, I know this isn't correct because I am supposed to show that in the last term of the question equation we have [tex]\frac{\partial u}{\partial x}[/tex] squared, rather than just [tex]\frac{\partial u}{\partial x}[/tex] as I would conclude.

    If my attempted solution doesn't make any sense, I'll try to clarify. But it is wrong either way and any help in finding the correct way to get the desired equation would be greatly appreciated.
  2. jcsd
  3. Feb 20, 2008 #2


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    It might be clearer if you write the whole thing in terms of f and g. But you are on the right track, it's product rule and chain rule put together. When you take another d/dx derivative of dz/du you will get the second derivative of z together with yet another du/dx in addition to the one you've already got, giving you a (du/dx)^2. Do you see that, or do I need to tex it?
  4. Feb 21, 2008 #3
    oh ok....so

    the derivative of [tex]\frac{dz}{du}[/tex] = [tex]\frac{d^{2} z}{du^{2}}[/tex] [tex]\frac{\partial u}{\partial x}[/tex]

    because we really are taking the derivative with respect to x.. so basically there was another chain that I didn't see..

    is that right?
  5. Feb 21, 2008 #4


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    I think so. There's a chain you didn't see all right. If you get what you were supposed to then you are doing it right.
  6. Feb 21, 2008 #5
    thanks a lot..
  7. Jun 28, 2010 #6
    Re: [SOLVED] Chain rule problem with partial derivatives

    Would anyone tex the solution in a step by step form please?
    Thank you in advance
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