What is the identity for coplanar, non-collinear vectors?

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Discussion Overview

The discussion revolves around the identity related to coplanar, non-collinear vectors and the conditions under which certain linear combinations of these vectors equal zero. Participants explore the implications of linear dependence and the relationships between coefficients in vector equations.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant recalls an identity stating that for coplanar, non-collinear vectors \(a, b, c\), if \(\alpha a + \beta b + \gamma c = 0\), then \(\alpha + \beta + \gamma = 0\).
  • Another participant challenges this by asserting that if \(a, b, c\) are coplanar, they are linearly dependent, and thus any third vector can be expressed as a linear combination of two basis vectors formed by \(a\) and \(b\).
  • Some participants express confusion about the conditions under which the identity holds, suggesting that the original statement should be modified to reflect non-coplanarity.
  • There is a clarification that if all coefficients are zero, then their sum is also zero, but this does not necessarily validate the original identity as stated.
  • Another participant emphasizes that changing the condition from coplanar to non-coplanar alters the validity of the statement.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the identity or its conditions. There are competing views regarding the implications of coplanarity and the relationships between the coefficients in the vector equation.

Contextual Notes

There are unresolved assumptions regarding the definitions of coplanarity and linear dependence, as well as the implications of modifying the conditions of the identity.

atavistic
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I kinda remember some identity which goes as follows:

If [tex]a,b,c[/tex] are coplanar, non collinear vectors then

[tex]\alpha a + \beta b + \gamma c = 0[/tex]
=> [tex]\alpha + \beta + \gamma = 0[/tex]

or something like this. Can someone help me remember.
 
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Well, this isn't quite correct.

If a, b, and c are three coplanar vectors, they are for sure linearly dependent in the plane, since, if a, b are two non colinear non zero vectors in a plane, they form a basis, i.e. [tex]\alpha a + \beta b = 0[/tex] => [tex]\alpha = \beta = 0[/tex]. Every third vector can be representet uniquely as a linear combination of the basis vectors a and b.
 


atavistic said:
I kinda remember some identity which goes as follows:

If [tex]a,b,c[/tex] are coplanar, non collinear vectors then

[tex]\alpha a + \beta b + \gamma c = 0[/tex]
=> [tex]\alpha + \beta + \gamma = 0[/tex]

or something like this. Can someone help me remember.

You need to replace "a, b, and c are coplanar" by "a, b, and c are NOT coplanar".
 


radou said:
Well, this isn't quite correct.

If a, b, and c are three coplanar vectors, they are for sure linearly dependent in the plane, since, if a, b are two non colinear non zero vectors in a plane, they form a basis, i.e. [tex]\alpha a + \beta b = 0[/tex] => [tex]\alpha = \beta = 0[/tex]. Every third vector can be representet uniquely as a linear combination of the basis vectors a and b.

Also for coplanar vectors
[tex]a\cdot(b\times c)= 0[/tex]
 


I am sorry but I really mean [tex]\alpha + \beta + \gamma = 0[/tex] and not [tex]\alpha = \beta =\gamma = 0[/tex]
 


atavistic said:
I am sorry but I really mean [tex]\alpha + \beta + \gamma = 0[/tex] and not [tex]\alpha = \beta =\gamma = 0[/tex]

Well, if [tex]\alpha = \beta =\gamma = 0[/tex], then most certainly [tex]\alpha + \beta + \gamma = 0[/tex]. :wink:
 


atavistic said:
I kinda remember some identity which goes as follows:

If [tex]a,b,c[/tex] are coplanar, non collinear vectors then

[tex]\alpha a + \beta b + \gamma c = 0[/tex]
=> [tex]\alpha + \beta + \gamma = 0[/tex]

or something like this. Can someone help me remember.

mathman said:
You need to replace "a, b, and c are coplanar" by "a, b, and c are NOT coplanar".
No, if they were not coplanar, that statement would not be true.
 


HallsofIvy said:
No, if they were not coplanar, that statement would not be true.

In his original statement he had all coef = 0, not the sum. Obviously changing the question would usually lead to a change in the response.
 


Oh, thanks. I hate it when people edit their post after there have been responses!
 

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