       Re: Q. How to work with derivative?

• To: mathgroup at smc.vnet.net
• Subject: [mg6676] Re: [mg6615] Q. How to work with derivative?
• From: Richard Finley <trfin at fiona.umsmed.edu>
• Date: Wed, 9 Apr 1997 09:15:44 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```Yaroslaw:

In:= f[x_]:=x^3
In:= D[Log[f[x]],x]/.x->3
Out=1

This is correct since the derivative of Log[x^3] is just 3/x which gives 1
when evaluated at x=3.
RF

At 11:07 PM 4/5/97 -0500, you wrote:
>If any one has any experience with the following, could you please
>help?
>
>Suppose I have a function f[x], say f[x]=x^3. Then I want to calculate
>the derivative of say Log[f[x]] and evaluate it at some point, say x=3.
>I try:
>--------------------------------------
>Clear[f,s];
>f[x_]:=x^3;
>s[x_]:=D[Log[f[x]],x];
>s
>General::"ivar": "\!\(3\) is not a valid variable."
>Out=
>\!\(\[PartialD]\_3 Log\)
>-------------------------------------
>Try onother approach:
>------------------------------------------------
>Clear[f];
>f[x_]:=x^3;
>Function[x,D[Log[f[x]],x]]
>General::"ivar": "\!\(3\) is not a valid variable."
>Out=
>\!\(\[PartialD]\_3 Log\)
>------------------------------------------------
>Both times my "3" goes into the notation for derivative, so
>it appears as if I want to differentiate with respect to "3".
>
>Of course I can do
>-----------------------------------------------
>Clear[f];
>f[x_]:=x^3;
>D[Log[f[x]],x]/.x->3
>
>Out[]=
>1
>-----------------------------------------------
>and get the result, but I need a _function_ (which I can later
>plot for instanse). What is the possible way to do it along the lines
>of my first approach?
>
>Thank you for all suggestions,
>Yaroslaw.
>
>

```

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