Perpendicular lines do not appear perpendicular

*To*: mathgroup at smc.vnet.net*Subject*: [mg100300] Perpendicular lines do not appear perpendicular*From*: Bill <WDWNORWALK at aol.com>*Date*: Sun, 31 May 2009 06:36:06 -0400 (EDT)

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.