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Re: problems plotting a derivative

  • To: mathgroup at
  • Subject: [mg54550] Re: problems plotting a derivative
  • From: Bill Rowe <readnewsciv at>
  • Date: Tue, 22 Feb 2005 04:25:41 -0500 (EST)
  • Sender: owner-wri-mathgroup at

On 2/21/05 at 3:44 AM, rob at (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.

When this happens, it is almost always a result of the way the plot routines evaluate arguements and is resolved by forceing evaluation of the expression before the plot routine gets it.

>I've tried using Evaluate[ D[f[x],x] ] 

This is close to what is needed. Try

ParametricPlot[Evaluate@{D[f[x], x], x^2}, {x, 0, 1}]
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