Linear algebra - set of solutions

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Homework Help Overview

The discussion revolves around a system of linear equations involving a parameter λ and whether this system can have a single solution, infinitely many solutions, or no solutions at all for different values of λ. Participants are exploring the implications of the rank of the matrix formed by the equations and the conditions under which solutions exist.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants are attempting to analyze the system by performing elementary row operations on the corresponding matrix and questioning the conditions under which the rank of the matrix relates to the number of solutions. There are inquiries about specific values of λ and their impact on the solution set.

Discussion Status

Some participants have provided insights based on their calculations and observations regarding the rank of the matrix, while others express confusion about the implications of their findings, particularly concerning the values of λ that yield different types of solutions. There is an ongoing exploration of the relationship between the determinant and the number of solutions, with no clear consensus reached yet.

Contextual Notes

One participant notes that they are not permitted to use knowledge of determinants, which may limit their ability to fully engage with the discussion. There are also references to specific values of λ (3 and 7/6) that are under scrutiny for their effect on the solution set.

peripatein
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Hello,

Is it true that the following system of linear equations would always have a single solution (i.e. would never have an infinite number of solutions nor none) for any value of λ?

λx + 3y -z = 1
x + 2y -z = 2
-λx + y + 2z = -1

May someone kindly confirm?
 
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How did you conclude that? What did you try?
 
Via a series of elementary operations I got:
lambda*x +7y = 1
(1/2)x + y -(1/2)z = 1
4y + z = 0

So, rank of the matrix is equal to the number of columns, 3, is it not? Wouldn't that always have a solution, for any lambda that is?
 
Alas, I am still experiencing difficulties trying to find the values of lambda for which number of solutions of the system of equations is: (i) 1; (ii) infinite; (iii) 0
Having substituted the equations into a matrix and performed a series of elementary operations on the rows, I have found out that there is a single solution for lambda different than 3 and 7/6. However, when lambda is 3 the system still has a single solution, hence a contradiction!
What am I doing wrong? I have tried various row operations several times, and each time upon substitution of those values for lambda (for which it couldn't possibly have a single solution) the system still has a single solution!
Would anyone please help with this?
 
peripatein said:
Via a series of elementary operations I got:
lambda*x +7y = 1
(1/2)x + y -(1/2)z = 1
4y + z = 0

So, rank of the matrix is equal to the number of columns, 3, is it not? Wouldn't that always have a solution, for any lambda that is?

For some values of λ the determinant of the coefficient matrix vanishes. What does that tell you?

RGV
 
We haven't dealt with determinants so I wouldn't know, and even if I did I am not permitted to use employ any knowledge thereof. Could you please just tell me what it is I might be doing wrong? For hours I have tried reducing the matrix and each time the lambda's I got for which the number of solutions would be one, ended up, upon substitution, yielding a solution! So I am pretty stuck and would appreciate some more practical guidance, please.
 

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