Re: problems plotting a derivative

*To*: mathgroup at smc.vnet.net*Subject*: [mg54545] Re: problems plotting a derivative*From*: Curt Fischer <tentrillion at gmail.NOSPAM.com>*Date*: Tue, 22 Feb 2005 04:25:18 -0500 (EST)*References*: <cvc80j$qqu$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

J. K. Smith 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)? f[x_] := 2*x^3; g[x_] = D[f[x], x]; ParametricPlot[{g[x], x^2}, {x, 0, 1}] Note that this approach doesn't work if you use delayed assignment := instead of immediate assignment = in the definition of g[x]. I'm not sure why using Evaluate doesn't work. -- Curt Fischer