Re: Q. How to work with derivative?
- To: mathgroup at smc.vnet.net
- Subject: [mg6648] Re: [mg6615] Q. How to work with derivative?
- From: Allan Hayes <hay at haystack.demon.co.uk>
- Date: Wed, 9 Apr 1997 00:34:11 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Yaroslaw Bazaliy <yar at leland.stanford.edu> [mg6615] Q. How to work with derivative? Writes as follows - full message appended below ******** >>>>>>>>>>>>> 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]\) ------------------------------------- <<<<<<<<<<< Yaroslav: Clear[f,s]; f[x_]:=x^3; s[x_] = D[Log[f[x]],x]; (*Set, not SetDelayed*) s[3] 1 Allan Hayes, hay at haystack.demon.co.uk http://www.haystack.demon.co.uk ************************************************* f 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.