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Re: Coords of intersecting lines...

  • To: mathgroup at
  • Subject: [mg6371] [mg6371] Re: [mg5746] Coords of intersecting lines...
  • From: Robert Pratt <rpratt at>
  • Date: Fri, 14 Mar 1997 14:54:03 -0500 (EST)
  • Sender: owner-wri-mathgroup at

Perhaps I am missing something, but it seems that your problem is 
ill-posed.  Given three points that lie on (unspecified) perpendicular 
lines, one cannot necessarily determine the intersection point of the two 
lines.  You must specify the lines.  For example, consider the three 
vertices (a, b, and c) of an equilateral triangle.  If you choose any two of 
these vertices (say a and b) to determine one of the lines, and take the 
line through the third vertex (c) perpendicular to the first line, this 
gives an intersection point that is the midpoint of a and b.  But if you 
choose different pairs of perpendicular lines through a, b, and c, you 
get different intersection points, namely the midpoints of a and c, and b 
and c, respectively.

Rob Pratt
Department of Mathematics
The University of North Carolina at Chapel Hill
CB# 3250, 331 Phillips Hall
Chapel Hill, NC  27599-3250

rpratt at

On Tue, 14 Jan 1997, Van der Poorten G. wrote:

> HI,
> I know this is a basic question, but after 15 years i tend to forget
> some things, specialy when you dont need math every day.
> Now, what am i looking for ?
> I'm trying to set up some equations on movement of cars and shock
> absorbers.( I'm actualy an 3D animator.)
> I got stuck with an over complicated formula when trying to find the
> intersectionpoint of 2 lines, only knowing 3 points a(x,y) b(x,y) c(x,y)
> and that both lines are perpendicular.
> Due to computation time, i must find the smallest formula possible.
> Can anyone help me on this?
> Btw, if you have information on automotive motion, plz let me know
> Van der Poorten Geert
> 3Digit
> Belgium

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