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Homework Help: F(x, y, z) ?

  1. Jun 22, 2010 #1
    f(x, y, z) ???

    Hey there,

    Sorry for the title, but i cannot find any appropriate one for it, here is my question!

    If f(x, y, z) = tanh (x+2y+3z) => fx =?(what is the derivative of f(x, y, z) with respect to x?)

    Actually, i know the answer but i do not get the idea! In case it might help, the answer is that: sech^2(x+2y+3z)

    So, as i said, can someone explain it to me? How can they reach this answer? Thanks!
  2. jcsd
  3. Jun 22, 2010 #2


    Staff: Mentor

    Re: f(x, y, z) ???

    What you want is the partial derivative with respect to x. And you'll need the chain rule here, which looks something like this.
    [tex]\frac{\partial tanh(u)}{\partial x} = \frac{\partial tanh(u)}{\partial u} \cdot \frac{\partial u}{\partial x}[/tex]
  4. Jun 22, 2010 #3
    Re: f(x, y, z) ???

    Oh, i see now, i made a dummy mistake. Thank you for your quick reply!
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