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MathGroup Archive 1995

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Re: Why gives Plot[GTK,{q,o,30}] negat. min. at q=15 ? (GTK:=1/3 q^2 - 10 q + 100)

  • Subject: [mg2191] Re: Why gives Plot[GTK,{q,o,30}] negat. min. at q=15 ? (GTK:=1/3 q^2 - 10 q + 100)
  • From: ianc (Ian Collier)
  • Date: Fri, 13 Oct 1995 06:24:41 GMT
  • Approved: usenet@wri.com
  • Distribution: local
  • Newsgroups: wri.mathgroup
  • Organization: Wolfram Research, Inc.
  • Sender: daemon at wri.com ( )

In article <45d1n0$63m at ralph.vnet.net>, txp at pi.net (JanHolland) wrote:

> Say GTK = ATC = Average Total Cost := 1/3 q^2 - 10 q + 100
> 
> When I give Plot[GTK,{q,0,30}] the resulting graph gives a minimum
> below to X-axis at about q=15
> 
> When I give q=15 and N[GTK] it evaluates correctly to GTK=25.
> 
> What am I doing wrong ?  Thanks for any effort you take !
> 
> ------------- GMT 16.15
> Jan Holland
> txp at pi.net
> "A fact is a broken dream"


I think that if you look carefully at the axes the plot is correct.

Mathematica is drawing the x axis as the line y =30.

By default Mathematica focusses on the "interesting" areas of the
plot. In this case, since the minimum is at 25 it crops the plot
just below 25.

Try

    Plot[GTK,{q,0,30},
        PlotRange ->{0,100}]

I hope this helps,

--Ian

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Ian Collier
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