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>*Reply-to*: murray at math.umass.edu

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? > > > Thanks in advance, > > 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