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

**References**:**problems plotting a derivative***From:*"J. K. Smith" <rob@pi-overe.com>