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Re: problems plotting a derivative


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


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