Proving Orthogonality of Planes and Lines in 3D Geometry

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Discussion Overview

The discussion revolves around proving the orthogonality of lines and planes in 3D geometry, specifically focusing on the relationships between three planes: A, B, and C. Participants explore geometric arguments and proofs related to the intersection of these planes and the lines formed by their intersections.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes that if plane C is orthogonal to both planes A and B, then the line formed by the intersection of plane C with plane B should be orthogonal to the line wx, which is the intersection of planes A and B.
  • Another participant suggests that the normal to plane C being orthogonal to the normals of planes A and B implies that the line along C's intersection with B must also be orthogonal to the intersection of A and B.
  • A specific example is provided using Cartesian coordinates to illustrate the planes and their intersections, with claims about the orthogonality of the lines involved.
  • Some participants express uncertainty about whether the argument holds in general or if it only applies to specific cases, questioning the validity of the proof in broader contexts.
  • There is a discussion about the nature of orthogonality in relation to lines within planes, with one participant asserting that the axiom regarding orthogonality does not apply universally to all lines in the planes.
  • Another participant offers a more formal proof involving vector geometry, suggesting that the orthogonality can be established through equations representing the planes and their intersections.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the proposed proofs and the axioms related to orthogonality. Some agree with the logic presented, while others challenge the assumptions made, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Limitations include the dependence on specific geometric configurations and the potential for misinterpretation of axioms related to orthogonality in 3D space. The discussion does not reach a consensus on the general applicability of the proofs provided.

  • #31
gnome said:
They're all just lines.

Oh, 'X' means cross product. I thought you meant something else by FXG, etc., the lack of spacing threw me.

Then what you wrote is axiomatic, and it follows from the fact that we're working in 3 dimensions. Simple enough ? :smile:
 
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  • #32
I guess. It just bothers me that I can't put my finger on any specific definition or postulate that says that.
 
  • #33
Draw a perpendicular from point w to the plane C. Any plane that contains that line is, by definition, perpendicular to C.
In particular, the plane that contains that perpendicular and the intersection of A and C. Since a point and a line define a plane, this plane must be plane A.
Now, consider the plane that contains the perpendicular and the intersection of B and C. By the same reasoning, this must be plane B.
So, the perpendicular from w to C belongs simultaneously to A and B and must be their intersection.
The intersection of A and B is orthogonal to all lines in C. QED.
 

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