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Equation of a line segment in 3D?

  1. Apr 17, 2015 #1
    Is there an equation for say a line segment in 3D Cartesian Coordinates that is between these two points?

    Point One: x=0, y=0, z=0
    Point Two: x=5, y=5, z=5

    Does it involve inequalities?
     
  2. jcsd
  3. Apr 17, 2015 #2

    robphy

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    For a segment, you need to somehow restrict the allowed values of x,y,z.
    So, you could require 0<x<5, 0<y<5, 0<z<5... for the line that passes through (0,0,0) and (5,5,5).
    You could also do it parametrically... but that too has a restriction (which could be interpreted as an inequality.. e.g.
    0<t<1, x=5t,y=5t,z=5t).

    Maybe, you could so some crazy parametrization where t runs from -infinity to infinity
    and asymptotically meets the end points... something akin to x=Aarctan(t),y=Barctan(t),z=Carctan(t), for all t.
     
    Last edited: Apr 17, 2015
  4. Apr 17, 2015 #3

    HallsofIvy

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    A one dimensional figure, such as a line or line segment cannot be written in a single equation in three dimensions. You can write three parametric equation in a single parameter or write it in "symmetric" form.

    The line through [itex](x_1, y_1, z_1)[/itex] and [itex](x_2, y_2, z_2)[/itex] can be written as
    [itex]x= (x_2- x_1)t+ x_1[/itex]
    [itex]y= (y_2- y_1)t+ y_1[/itex]
    [itex]z= (z_2- z_1)t+ z_1[/itex]

    To write it in "symmetric" form, solve each of those for t,
    [tex]t= \frac{x- x_1}{x_1- x_1}[/tex]
    [tex]t = \frac{y- y_1}{y_2- y_1}[/tex]
    [tex]t= \frac{z- z_1}{z_2- z_1}[/tex]
    and set them equal:
    [tex]\frac{x- x_1}{x_1- x_1}= \frac{y- y_1}{y_2- y_1}= \frac{z- z_1}{z_2- z_1}[/tex]
     
  5. Apr 17, 2015 #4
    [itex]\vec x (t) = \vec{x}_1 + (\vec{x}_2 - \vec{x}_1)f(t)[/itex] where [itex]0 \leq f(t) \leq 1[/itex].

    If say [itex]f(t) = \frac{1}{\pi}\arctan(t) + \frac{1}{2}[/itex], then you've covered the entire real line.

    As for an equation, [itex]\vec{x}_1 + (\vec{x}_2 - \vec{x}_1)(\frac{1}{\pi}\arctan(t) + \frac{1}{2}) - \vec x = 0[/itex] works, yes?
     
    Last edited: Apr 17, 2015
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