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Re: problems plotting a derivative
*To*: mathgroup at smc.vnet.net
*Subject*: [mg54530] Re: [mg54479] problems plotting a derivative
*From*: DrBob <drbob at bigfoot.com>
*Date*: Tue, 22 Feb 2005 04:24:03 -0500 (EST)
*References*: <200502210844.DAA27207@smc.vnet.net>
*Reply-to*: drbob at bigfoot.com
*Sender*: owner-wri-mathgroup at wolfram.com
This works:
f[x_] := 2*x^3;
ParametricPlot[Evaluate[
{D[f[x], x], x^2}], {x, 0, 1}];
and this works:
{D[f[x], x], x^2};
ParametricPlot[%, {x, 0, 1}];
Both methods require x being an undefined symbol (it can't have a value).
You may have tried this, which DOESN'T work:
ParametricPlot[{Evaluate@D[f[x], x], x^2}, {x, 0, 1}]
That doesn't work, because D[f[x],x] isn't the Held argument of ParametricPlot; the List that contains it is Held, so Evaluate (in that example) isn't reached until x has a value, and D complains that you're differentiating with respect to a constant.
Bobby
On Mon, 21 Feb 2005 03:44:47 -0500 (EST), J. K. Smith <rob at pi-overe.com> wrote:
> I've tried a lot of ways but nothing has worked so far.
> The following statements are my problem simplified:
>
> f[x_] := 2x^3;
> ParametricPlot[{D[f[x], x], x^2}, {x, 0, 1}]
>
> When I run this I get error messages griping about numbers not being
> valid variables but it doesn't end up plotting anything. In this example
> I should see a straight line.
>
> I've tried using Evaluate[ D[f[x],x] ] and lots of other schemes which
> seemed to work in the past but I can't get ParametricPlot to digest any
> of them. Something about the result of that derivative doesn't yield
> something that can be plotted. But I don't know what property it is.
>
> Would you kind Mathematica gurus please get me thru this one (too)?
>
> Thanks.
>
> remove the dash if emailing
>
>
>
>
--
DrBob at bigfoot.com
www.eclecticdreams.net
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