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MathGroup Archive 2007

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Re: What to do in v. 6 in place of

  • To: mathgroup at
  • Subject: [mg76829] Re: What to do in v. 6 in place of
  • From: Helen Read <hpr at>
  • Date: Mon, 28 May 2007 01:00:12 -0400 (EDT)
  • References: <f3bh1e$37g$>

Bob Hanlon wrote:
> This works for simple cases such as your examples.
> Clear[realPlot];
> realPlot[expr_,
>    {x_Symbol, xmin_?NumericQ, xmax_?NumericQ},
>    opts___] := Module[{d, e, f, y},
>    e = Flatten@{expr};
>    d = (Denominator@PowerExpand@Log[x, #] & /@ e);
>    f = y /. (ToRules@
>          Reduce[y^#[[2]] == PowerExpand[#[[1]]^#[[2]]],
>           y, Reals] & /@ Thread[{e, d}]);
>    Plot[f, {x, xmin, xmax}, opts]];
> Grid[{{realPlot[x^(1/3), {x, -8, 8}],
>    realPlot[x^(3/5), {x, -8, 8}]},
>   {realPlot[{x^(1/3), x^(3/5)}, {x, -8, 8}]}}]

This completely misses the point, which is to make it easy for beginning 
calculus students to plot functions such as x^(1/3) for negative x. 
There is no way they could follow what you have written, let alone 
reproduce it themselves.

Probably the best solution at this point is to simply use the old 
Miscellaneous`RealOnly` package when this comes up in class, and just 
ignore the "obsolete package" message.

Helen Read
University of Vermont

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