Re: How can draw Focus(Points)

• To: mathgroup at smc.vnet.net
• Subject: [mg17151] Re: [mg17139] How can draw Focus(Points)
• From: Manuel Arala Chaves <machaves at fc.up.pt>
• Date: Sun, 18 Apr 1999 00:59:40 -0400
• Sender: owner-wri-mathgroup at wolfram.com

```
On Sat, 17 Apr 1999, Clara wrote:

> Hello
>
> I am beginner in Mathematica. I have created the parabola using:
> Plot1= Plot[ {y = 1/2x^2, y = -4}, {x, -4, 4},
> AxesLabel -> {"xx","yy"}
> , PlotStyle -> Hue[160/255],
> AxesStyle -> Hue [160/255] ]
>
> me.
>
>
>

Focus=Graphics[{PointSize[.02],Hue[160/255],Point[{0,.5}]}];
Show[Plot1,Focus, AspectRatio->Automatic]

But if your line under the parabola is supposed to be the directrix, there
is a mistake; the following will draw the parabola plus both focus and
directrix:

Plot2= Plot[ {0.5 x^2, -0.5}, {x, -4, 4}, AxesLabel -> {"xx","yy"},
PlotStyle -> Hue[160/255], AxesStyle -> Hue [160/255],
DisplayFunction->Identity ];
Focus=Graphics[{PointSize[.02],Hue[160/255],Point[{0,.5}]}];
Show[Plot2,Focus, AspectRatio->Automatic,
DisplayFunction->\$DisplayFunction]

================================================
Manuel Arala Chaves  -  Dep. of Pure Mathematics
Faculty of Science at the University of Porto
Pr. Gomes Teixeira  -  4050  Porto - PORTUGAL
***********
Tel.: Office-(351)(2)3401434; Fac-(351)(2)3401400
Fax Fac.(351)(2)2008628  Email: machaves at fc.up.pt
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Atractor - Interactive Mathematics
http://www.fc.up.pt/atractor

```

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