       Re: Q. How to work with derivative?

• To: mathgroup at smc.vnet.net
• Subject: [mg6681] Re: [mg6615] Q. How to work with derivative?
• From: "Preferred Customer" <sherman.reed at worldnet.att.net>
• Date: Thu, 10 Apr 1997 01:08:59 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```
f[x_]:=x^3
s[x_]=D[Log[f[x]],x];
s

removing the delayed definition of the derivative solves the problem.
there is probably another way, but this is straightforward.
sherman reed
----------
> From: Yaroslaw Bazaliy <yar at leland.stanford.edu>
To: mathgroup at smc.vnet.net
> To: mathgroup at smc.vnet.net
> Subject: [mg6681] [mg6615] Q. How to work with derivative?
> Date: Saturday, April 05, 1997 11:07 PM
>
> 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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