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MathGroup Archive 2001

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Re: simple Problem: D[ ]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27331] Re: simple Problem: D[ ]
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Wed, 21 Feb 2001 03:17:06 -0500 (EST)
  • References: <96t91a$pqt@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Christian,
It works OK for me

f[x]=x^2

        x^2

abl=D[f[x],x]

        2 x

But all that happens here is that  D[f[x],x] changes to D[x^2,x] and then to
2x.
We are actually differentiation an expression in  x, not a function.

The function approach is

g[x_]:= x^2

g'[x]

        2 x

g'[5]

        10

But beware,

D[g[a],x]

        0
since g[a] is a^2 with no x in it.

--
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565

"Christian Maier" <maier1 at sbox.tu-graz.ac.at> wrote in message
news:96t91a$pqt at smc.vnet.net...
>
> I use mathmatica the first time and have a problem with a deriviation:
>
> in a simple problem:
>
> f[x]=x^2
> abl=D[f[x],x]
>
> then Mathematica puts out f[x]'    , but not 2 x
>
> How does it work correctly?
> Thank aou in advance!
>
> Christian Maier
>
>
>




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