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: Here is an answer to your question: In[1]:= f[x_]:=x^3 In[3]:= D[Log[f[x]],x]/.x->3 Out[3]=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[3] >General::"ivar": "\!\(3\) is not a valid variable." >Out[47]= >\!\(\[PartialD]\_3 Log[27]\) >------------------------------------- >Try onother approach: >------------------------------------------------ >Clear[f]; >f[x_]:=x^3; >Function[x,D[Log[f[x]],x]][3] >General::"ivar": "\!\(3\) is not a valid variable." >Out[48]= >\!\(\[PartialD]\_3 Log[27]\) >------------------------------------------------ >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. > >