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Partial derivatives

  1. Jun 20, 2005 #1
    hello, i am supposed to use the two variable chain rule to confirm that changing variables from (x,y) to (v,w) with v=x and w=y/x leads to:

    \partial{v}=\partial{x} + w\partial{y}
    and \partial{w}=x\partial{y}

    it seems to me that the first line should read \partial{v}=\partial{x} = w\partial{y}

    is this true? i am a bit rusty on multivariable calculus
     
  2. jcsd
  3. Jun 20, 2005 #2

    dextercioby

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    Please,use the [ tex ] and [ /tex ] tags (without the spaces,of course).It doesn't make too much sense what you've written there.

    Usually [itex] \partial D[/itex] all by himself means the boundary of the domain D.

    Daniel.
     
  4. Jun 20, 2005 #3
    for this to make any sense, you need to express it as a function, say [itex]f(x,y)[/itex] and then take derivatives of [itex]f[/itex]
     
  5. Jun 20, 2005 #4
    ok, here is a repost

    hello, i am supposed to use the two variable chain rule to confirm that changing variables from (x,y) to (v,w) with v=x and w=y/x leads to:

    [itex]\frac{\partial}{\partial v} = \frac{\partial}{\partial x} + w\frac{\partial}{\partial y} [/itex]
    and [itex]\frac{\partial}{\partial w}=x\frac{\partial}{\partial y}[/itex]

    it seems to me that the first line should read [itex]\frac{\partial}{\partial v} = \frac{\partial}{\partial x} = w\frac{\partial}{\partial y} [/itex]

    is this true? i am a bit rusty on multivariable calculus
     
  6. Jun 20, 2005 #5

    dextercioby

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    Hmmm

    [tex] \frac{\partial}{\partial v}=\frac{\partial x}{\partial v}\frac{\partial}{\partial x}+
    \frac{\partial y}{\partial v}\frac{\partial}{\partial y} [/tex]

    [tex] \frac{\partial}{\partial w}=\frac{\partial x}{\partial w}\frac{\partial}{\partial x}+\frac{\partial y}{\partial w}\frac{\partial}{\partial y} [/tex]

    There's no other way to apply the chain rule.

    You could put it as well in matrix notation using the Jacobian matrix.

    Daniel.
     
  7. Jun 20, 2005 #6
    ok, thanks
    i can see where the first one comes from, but if i apply the chain rule to the second one, i get

    [itex]\frac{\partial}{\partial w}=\frac{-y}{w^2}\frac{\partial}{\partial x} + x\frac{\partial}{\partial y}[/itex]

    is this correct, or is the original correct?
     
    Last edited: Jun 20, 2005
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