Interseccion of two planes in R3

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Discussion Overview

The discussion centers on how to find the intersection of two planes in three-dimensional space (R3). Participants explore the mathematical representation of planes and the implications of their intersection, including the determination of a line defined by the intersection of two planes.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asks how to find the intersection of two planes in R3, suggesting the use of the cross product of the normal vectors to determine a direction vector.
  • Another participant provides a definition of a plane in R3, indicating that it is defined by a normal vector and a point, and can be expressed in the form ax+by+cz=d.
  • A subsequent reply emphasizes that the general linear form describes a plane, not a line, and clarifies that two planes intersect to form a line, which requires two equations to define.
  • It is noted that any linear combination of the equations of the two planes will also describe the intersection line, introducing the parameter λ to represent this relationship.
  • Participants mention terms such as "pencil of planes," "fan of planes," and "sheaf of planes" in relation to the intersection of planes.

Areas of Agreement / Disagreement

Participants generally agree on the definitions of planes and the nature of their intersection, but there are nuances regarding the representation of lines and the equations involved that remain contested.

Contextual Notes

The discussion does not resolve the specific methods for finding a point of intersection or the implications of different representations of lines in three-dimensional space.

Kiwiro0ls
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How do you find the intersection of two planes in R3? The direction vector would be the cross product between the two normal vectors I imagine. So, how do I go about finding a point that lies in both planes so I can find the equation of the line?

Thanks :)
 
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What do you know about the equation of a plane in R3?
 
A plane is defined by a normal vector and a point. It can be written as
ax+by+cz=d where (a,b,c) is the normal vector and d is <(x1,y1,z1),(a,b,c)>
 
It can be written as
ax+by+cz=d

Exactly so.

The general linear form in 3D is a plane, not a line.

In fact there is no single "equation of a line in 3D", which is probably why you can't find one.

Two planes intersect in a line so a line is defined by two planes.

A line has to be defined by two equations, not one.

a1x+b1y+c1z=d1 = P1
a2x+b2y+c2z=d2 = P2

For an alternative pair of equations see here

https://www.physicsforums.com/showthread.php?t=641057

However any linear combination of P1 and P2 will also contain this line. This can be described by the parameter λ such that

P1 + λ(P2) = 0

Edit:
This is referred to as a pencil of planes or a fan of planes or a sheaf of planes.

Wolfram have a good picture.
http://mathworld.wolfram.com/SheafofPlanes.html
 
Last edited:

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