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Re: How to plot derivative directly?

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On Apr 11, 7:05 am, =A9er=FDch Jakub <Ser...@panska.cz> wrote:
> Dear mathgroup,
>
> it seems to me, that response to my question shall be very simple,
> but I cannot find it. :-(
>
> I want to plot the derivative of the function. I would like to do it
> directly, something like:
>
> Plot[D[x^3 - 6 (x + 1)^2 + x - 7, x],{x,-3,8}]
>
> It returns: General::ivar: "-2.99978 is not a valid variable."
>
> I can understand that it is because local variable x from Plot command
> interferes with the x variable from the D[].
>
> Yes I can bypass the problem by:
> deriv = D[x^3 - 6 (x + 1)^2 + x - 7, x]
> Plot[deriv, {x, -3, 8}]
>
> which is fully functional, but as far as I know Mathematica, there must
> be some simple solution how to do it directly inside the Plot[].
>
> Thanks in advance for kick-off
>
> Jakub

Jakub, the Plot[] function picks a set of discrete x values (within
the range specified, very closely spaced to create the smooth graph)
and tries to evaluate your function with those numbers in place of the
x's. This creates a problem when it then tries to evaluate
D[-3^3-6(-3+1)^2+(-3)-7, -3], since you cant differentiate with
respect to a number like -3, hence the 'General::ivar: "-2.99978 is
not a valid variable." ' error message. You can get it to evaluate the
derivative before substituting in numbers for x by simply enclosing
your derivative inside an Evaluate[]

Plot[Evaluate[D[x^3 - 6 (x + 1)^2 + x - 7, x]], {x, -3, 8}]

Hope that helps!

-Stefan

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Thread

How to plot derivative directly? Šerých Jakub <Serych@panska.cz> - 2011-04-11 11:05 +0000
  Re: How to plot derivative directly? Peter Pein <petsie@dordos.net> - 2011-04-12 09:53 +0000
  Re: How to plot derivative directly? Helen Read <readhpr@gmail.com> - 2011-04-12 09:54 +0000
  Re: How to plot derivative directly? Stefan <wutchamacallit27@gmail.com> - 2011-04-12 09:56 +0000
  Re: How to plot derivative directly? Wolfgang Windsteiger <Wolfgang.Windsteiger@risc.jku.at> - 2011-04-12 09:57 +0000
  Re: How to plot derivative directly? Peter Breitfeld <phbrf@t-online.de> - 2011-04-12 09:58 +0000

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