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

• To: mathgroup at smc.vnet.net
• 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 02:24:41 -0400
• Organization: Wolfram Research, Inc.

```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

-----------------------------------------------------------
Ian Collier
Technical Sales Support
Wolfram Research, Inc.
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