Writing orthogonal vectors as linear combinations

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Discussion Overview

The discussion revolves around the possibility of expressing a vector \( c \) as a linear combination of two other vectors \( a \) and \( b \), given that \( a \) and \( c \) are orthogonal. Participants explore the conditions under which this is feasible, considering the relationships between the vectors involved.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions whether it is possible to write \( c \) as \( c = ma + nb \) given that \( a \) and \( c \) are orthogonal.
  • Another participant asserts that since \( a \) and \( c \) define a plane, there exists a vector \( b \) such that \( c = ma + nb \) lies in that plane, suggesting it is possible.
  • A different participant seeks clarification on whether the orthogonality condition applies to \( a \) and \( b \) instead, providing a formula for the coefficients if that were the case.
  • The original poster confirms the orthogonality of \( a \) and \( c \) and acknowledges that defining \( b \) in terms of \( a \) and \( c \) allows for rearranging to express \( c \) as a linear combination.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which \( c \) can be expressed as a linear combination of \( a \) and \( b\). While some suggest it is possible, others highlight the need for specific relationships between the vectors, indicating that the discussion remains unresolved.

Contextual Notes

There are assumptions regarding the relationships between the vectors \( a \), \( b \), and \( c \) that are not fully articulated, particularly concerning collinearity and orthogonality. The discussion does not resolve these dependencies.

thedemon13666
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Hello,

Quick question, not really homework but more of a general inquiry. Take three vectors: a,b and c such that a and c are orthogonal. Is it possible to write c as a linear combination of a and b such that:

c = ma + nb where m,n are scalars.

I was thinking not at first glance but reading around has made me think twice.
Is it possible?

Thanks
 
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Vectors a and c define a plane. There is definitely a vector b such that c=ma+nb that lies in that plane. So yes it is possible
 
thedemon13666 said:
Take three vectors: a,b and c such that a and c are orthogonal. Is it possible to write c as a linear combination of a and b such that:
c = ma + nb where m,n are scalars.
Did you mean, s.t. a and b are orthogonal? Yes: c = (a.c/a.a)a + (a.b/b.b)b
But if you meant a and c are orthogonal, not if a and b are collinear.
If a and b are neither collinear nor orthogonal, there's a range of solutions for the coefficients.
 
I actually mean a and c are orthogonal.

I see now, as it is possible to define the vector b in terms of a and c, it is then just rearranging.

Cheers!
 

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