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Partial derivatives

  1. Sep 26, 2009 #1
    Hi everyone!

    I was wondering if someone could help me with the following question with partial derivatives.

    A function f: R^2 -> R is defined by f(x,y) = g(x-2y), where g: R-> R.
    If g'(1)= 3, calculate f subscript x (3,1) and f subscript y of (3,1).

  2. jcsd
  3. Sep 26, 2009 #2


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    You can use the little X2 icon to type fx and fy.

    Do you know the chain rule for this setup:

    g is a function of u and u is a function of x and y, then

    gx = ? and gy = ?

    That's what you need.
  4. Sep 26, 2009 #3


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    hi jessica21, welcome to pf ;) usually it helps if you show some working...

    i would start by trying to find the partial derviatives of f & thinking about what a partial derviative means

    [tex] f_x = \frac{\partial f}{\partial x} = \frac{\partial }{\partial x} g(x-2y) [/tex]
  5. Sep 26, 2009 #4


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    also remember a partial derivative means the other variables are kept constant in the differentiation
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