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Re: simple Problem: D[ ]


With my release (4.0) I get 2x, but of course if I ask for f[3] I get
f[3] and not what expected (9).

One solution is to define f in this way:
f[x_] := x^2

If you are a mathematician, just think that x_ design a mathematical
variable that can be replaced by what you want. If you are a computer
man, then think that x_ is a filter of name x. In other cases, remember
that in Mathematica, if you want a symbol to be replaced by something
sometimes later, always use x_ or else think that you will have to
operate on x substitutions using rules like x-> something.

Jacqueline Zizi

Christian Maier wrote:

> 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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