Simple vector arithmetic question

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Discussion Overview

The discussion revolves around a question regarding vector arithmetic, specifically the relationship between two vectors that are 45 degrees apart and a resultant vector derived from them. Participants explore the conditions under which the resultant vector is orthogonal to one of the original vectors.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant posits that if vectors a and c have an angle of 45 degrees between them, then the vector b, defined as b = c - a, should be orthogonal to a.
  • Another participant suggests sketching vectors to find a counterexample to the initial claim.
  • A participant provides specific vector examples and calculates the resultant vector b, noting that it is not orthogonal to a despite the initial assumption.
  • The same participant expresses confusion over why b is not orthogonal to a, despite the vectors being defined under the given conditions.
  • One participant later resolves their confusion by acknowledging a mistake made in normalizing the resultant vector.
  • Another participant comments on the geometric interpretation of the vectors, noting that they form a triangle with one angle at 45 degrees and that the other angles must sum to 135 degrees.

Areas of Agreement / Disagreement

The discussion includes both agreement and disagreement, with some participants questioning the initial assumptions about orthogonality while others provide clarifications and corrections. The resolution of confusion by one participant indicates some level of agreement on the importance of normalization in vector calculations, but overall, multiple competing views remain regarding the properties of the vectors.

Contextual Notes

Participants have not fully resolved the mathematical implications of the vector relationships, particularly regarding the conditions for orthogonality and the geometric interpretation of the vectors involved.

The_Engineer
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I have two vectors: a = <ax, ay, az> and c = <cx, cy, cz>

which have an angle of 45 degrees between them.


If I get another vector by b = c - a then shouldn't b be orthogonal to a? I'm assuming this since a + b = c
 
Last edited:
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Did you try to sketch some vectors and to come up with a counter example?
 
A.T. said:
Did you try to sketch some vectors and to come up with a counter example?

Yes I have been using MATLAB to sketch and generate random examples. Here is one...

a = <0.3814, 0.9023, 0.2010>
c = <0.3965, 0.7378, -0.5463>

The angle between these vectors is 45 degrees.

I want a vector b such that b is orthogonal to a AND 45 degrees from c. Graphically speaking, this means a + b = c

Solving for b,
b = c - a = <0.0151, -0.1645, -0.7473>.



But b dot a ≠ 0 therefore they aren't orthogonal. Why aren't a and b orthogonal?


This happens for every random sample I make, starting with 2 vectors that are 45 degrees apart.
 
Nevermind I figured it out... I made a mistake by normalizing the resultant, which in turn messed up my final answer.
 
The vectors you describe will form a triangle. With one angle at 45 deg. the other two angles could be anything that add up to 135 deg.
 

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