Two linearly independent vectors in a plane that don't span the plane

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Homework Help Overview

The discussion revolves around the properties of vectors in the context of a plane defined by the equation x+2y+4z=8. Participants explore the concept of linear independence and spanning sets within this plane, questioning the implications of having vectors that do not span the space.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants examine the linear independence of vectors (8,0,0) and (0,0,2) within the plane and question how these vectors can fail to span the plane when they are linearly independent. There is also a discussion about the nature of the plane as a subspace and the implications of it not passing through the origin.

Discussion Status

Some participants have provided insights regarding the nature of the plane not being a subspace, which affects the spanning property. There is acknowledgment of the misunderstanding about the vectors being in the plane versus being position vectors to points in the plane.

Contextual Notes

Participants note that the plane defined by the equation does not include the origin, which is a critical aspect of the discussion regarding vector spaces and spanning sets.

johnqwertyful
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Homework Statement



Say we have the plane, x+2y+4z=8 (part of a larger problem)

Homework Equations


The Attempt at a Solution



The vectors (8,0,0) and (0,0,2) both lie in the plane. They are linearly independent. But (0,4,0) lies in the plane and is not a linear combination of the first two vectors. How can this be? We have two linearly independent vectors in a two dimensional vector space that DON'T span it?
 
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johnqwertyful said:

Homework Statement



Say we have the plane, x+2y+4z=8 (part of a larger problem)

Homework Equations





The Attempt at a Solution



The vectors (8,0,0) and (0,0,2) both lie in the plane. They are linearly independent. But (0,4,0) lies in the plane and is not a linear combination of the first two vectors. How can this be? We have two linearly independent vectors in a two dimensional vector space that DON'T span it?

The plane you have is not a subspace. A subspace would need to pass through the origin, so would need to have '0' on the right, not your '8'. Since you do not have a subspace, there is no reason to have the spanning property you want. Just draw a picture to see what is happening.
 
(0,0,0) is not in the plane, so this is not a vector space. Nevermind.
 
Ray Vickson said:
The plane you have is not a subspace. A subspace would need to pass through the origin, so would need to have '0' on the right, not your '8'. Since you do not have a subspace, there is no reason to have the spanning property you want. Just draw a picture to see what is happening.

Figured it out just after I posted, thanks though!
 
johnqwertyful said:

Homework Statement



Say we have the plane, x+2y+4z=8 (part of a larger problem)

Homework Equations





The Attempt at a Solution



The vectors (8,0,0) and (0,0,2) both lie in the plane. They are linearly independent. But (0,4,0) lies in the plane and is not a linear combination of the first two vectors. How can this be? We have two linearly independent vectors in a two dimensional vector space that DON'T span it?

None of your three vectors lie in the plane. They are points in the plane. You can also think of them as position vectors to those points, which is why they aren't in the plane.
 

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