       Re: Perpendicular lines do not appear perpendicular

• To: mathgroup at smc.vnet.net
• Subject: [mg100369] Re: [mg100300] Perpendicular lines do not appear perpendicular
• From: Murray Eisenberg <murray at math.umass.edu>
• Date: Tue, 2 Jun 2009 06:42:23 -0400 (EDT)
• Organization: Mathematics & Statistics, Univ. of Mass./Amherst
• References: <200905311036.GAA16964@smc.vnet.net>

```The two lines pass the square-piece-of-paper test on my LCD monitor.

Bill wrote:
> Perpendicular lines do not appear perpendicular
>
> Hi:
>
> I have taken the line equation 2*x+4*y-6==0, and rotated it 90 degrees counterclockwise about {0,0},
> using a 3x3 rotation matrix.
> Here's the Mathematica 6.0.1 code that I used:
>
> rMat = {{Cos[90 Degree], - Sin[90 Degree], 0}, {Sin[90 Degree],
> Cos[90 Degree], 0}, {0, 0, 1}}
>
> l={2, 4, -6}
>
> rMat.l
>
> Output: {-4,2,-6}
>
> Placing these numbers into the general form of the equation of the line Ax+By+c=0, I get -4*x + 2*y - 6 == 0.
> Placing the 2 perpendicular line equations into ContourPlot:
>
> ContourPlot[{2*x + 4*y - 6 == 0, -4*x + 2*y - 6 == 0}, {x, -4,
>   4}, {y, -4, 4}, AspectRatio -> 1]
>
> and plotting, I notice that on my pc screen that the lines do not look perpendicular, as does the plot frame.
> Perhaps this is just my computer screen, a HP f2105. I can hold up a square cut piece of paper to the lines
> on the screen, and notice the out-of-square condition. I used AspectRatio->1, but still no joy.
>
> Question: Can someone suggest a remedy for my problem, if one exists?
>
>
>
> Bill
>
> PS. Mathematica 6.0.1 on a pc with Win XP.
>

--
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305

```

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