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Homework Help: Need Help Understanding an Equality

  1. Oct 2, 2013 #1
    I'm mostly trying to understand how the expression in brackets becomes a partial derivative.

    I know I end up with 0/0 if I replace all deltaz with 0, so that leads me to belive L'hopitals rule is used here. I just don't see how I could take L hopitals when I have two variables..

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  3. Oct 3, 2013 #2

    Simon Bridge

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    Take the limit as Δz→0 and the LHS becomes the definition for the partial derivative.
    Last edited: Oct 3, 2013
  4. Oct 3, 2013 #3
    ? But wouldn't that give me 0/0 if I take the limit directly?
  5. Oct 3, 2013 #4

    Simon Bridge

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    How? Where?

    I have to go ... so, notice:
    ##\Delta z## only appears on the LHS, so the limit dos not affect the RHS at all.
    It appears inside the function ##v(z,t)## in the numerator.
    How is this different from a regular derivative with only one variable?
  6. Oct 3, 2013 #5
    delta z is in the numerator and in the denominator on the LHS. I'm not sure what you're trying to say.
  7. Oct 3, 2013 #6

    Simon Bridge

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    Ahhh... I think I get where you are coming from.

    The fact that delta-z appears inside a function in the numerator makes all the difference.
    remember, the limit is what happens as delta-z gets very close to zero ... not what happens when delta-z actually is equal to zero. delta-z never reaches zero.

    I'm guessing you've not used this sort of expression before:
    $$\frac{d}{dx}f(x)=\lim_{h\rightarrow 0}\frac{f(x+h)-f(x)}{h}$$

    You could imagine that in the limit, f(x+h)=f(x), which makes the expression evaluate to 0/0.
    But that does not work for all possible f(x) - I only need one counter-example to show it's not always true, watch:

    example: f(x)=x
    $$\lim_{h\rightarrow 0}\frac{f(x+h)-f(x)}{h}= \lim_{h\rightarrow 0}\left [ \frac{(x+h)-x}{h}\right]=\lim_{h\rightarrow 0}\left[ \frac{h}{h}\right]=\lim_{h\rightarrow 0}[1]=1$$ ... see? Since all the h's cancel out, the limit evaluates into something sensible.

    The trick is to do the algebra with the aim of getting the h out of the denominator. Otherwise that could be a problem.

    example f(x)=x^2 ... try it.

    Now back to your example - put v(z,t)=z/t in the LHS, and take the limit delta-z approaches zero.
    Last edited: Oct 3, 2013
  8. Oct 3, 2013 #7


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    Surely, if you are looking at problems like this, you have taken Calculus? The definition of the "derivative of f(x) at x= a" is
    [tex]\lim_{h\to 0}\frac{f(a+ h)- f(a)}{h}[/tex]

    Obviously, if you simple "set h equal to 0" you will get the meaningless "0/0". That's the whole point of using the "limit". Taking the limit "as h goes to 0" is NOT the same as setting h= 0. You should have learned that in introductory Calculus.
  9. Oct 3, 2013 #8
    I end up with

    { [ (z/t) + Δz] - (z/t) } / Δz


    Δz / Δz



    then taking the limit I get


    Or am I supposed to do

    {[(z + Δz)/t] - (z/t)} / Δz

    [(z/t) + (Δz/t) -(z/t)] / Δz

    (Δz/t) / Δz


    How does this get me to the partial derivative with respect to z and how would I know that doing all this wouldn't give me a partial with respect to t instead?

    Also, was (z/t) arbitrary? Or would something like (zt) have worked?
  10. Oct 3, 2013 #9

    Simon Bridge

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    You use the normal rules for substitution:

    If ##v(z,t)=z/t##, then ##v(z+Δz,t) = (z+Δz)/t##

    i.e every time you see a z, you replace it with z+Δz

    So you have the LHS limit = 1/t - compare that with the partial derivative.
    (Do you know how to take a partial derivative?)

    The partial derivative of ##v(z,t)## with respect to ##t## would be:
    $$\frac{\partial}{\partial t}v(z,t)=\lim_{\Delta t\rightarrow 0}\frac{v(z,t+\Delta t)-v(z,t)}{\Delta t}$$

    In general,
    If you have a function ##f(\vec{x})## of ##N## variables ##\vec{x}=(x_1,x_2,\cdots ,x_N)##
    Then the partial derivative wrt the nth variable, where ##1\leq n \leq N##, is:
    $$\frac{\partial}{\partial x_n}f(\vec{x})=\lim_{\Delta x_n\rightarrow 0}\frac{f(\cdots, x_n+\Delta x_n,\cdots)-f(\cdots,x_n,\cdots)}{\Delta x_n}$$
    ... note: if you don't understand that notation you have a serious problem!

    Do it and see :)

    The definition works for any differentiable function. It would have worked for ##v(z,t)=z## or ##v(z,t)=t## ... whatever.

    Do you know how to differentiate functions by rule instead of using the definition?
    i.e. can you do: $$\frac{d}{dx}(ax^2+bx+e^{cx})$$
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