How Can We Construct a Unique Linear System From Given Conditions?

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Discussion Overview

The discussion revolves around constructing a linear system based on specific numerical conditions. Participants explore the implications of these conditions on the uniqueness of solutions, particularly focusing on a system defined by four numbers with given sums.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant presents a linear system defined by the equations: x1 + x2 + x3 + x4 = 40, x1 + x2 + x3 = 20, and x3 + x3 + x4 = 30, seeking help with the problem.
  • Another participant corrects the last equation to x2 + x3 + x4 = 30 and discusses the implications of the equations on the uniqueness of solutions.
  • There is a suggestion that by subtracting one equation from another, certain variables can be uniquely determined, but questions remain about the flexibility of choosing values for x3.
  • One participant proposes letting x3 = 10 and questions whether this choice allows for the determination of x2 and x1.
  • Another participant confirms that x3 can indeed be any number, leading to the conclusion that the system has infinitely many solutions.

Areas of Agreement / Disagreement

Participants generally agree that the system has infinitely many solutions based on the conditions provided, but there is ongoing discussion about the implications of specific choices for the variable x3.

Contextual Notes

Some assumptions about the nature of the variables and their relationships remain unresolved, particularly regarding the conditions necessary for achieving a unique solution.

Who May Find This Useful

This discussion may be useful for individuals interested in linear algebra, particularly those exploring systems of equations and the conditions that affect the uniqueness of their solutions.

orgekas
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Construct a linear system determined by four numbers whose sum is 40, with the first three numbers adding up to 20 and the last three to 30.

a) Explain why this system has infinitely many solutions.
b) Add another condition on the numbers so that a unique solution can be found and then find this solution.

Anyone can help?
Thanks.
 
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Please see rule #11 http://mathhelpboards.com/rules/ (click on the Expand button at the top of the list).

What do you know about systems of linear equations?
 
Ok sorry for not showing my work.

But I didn't have much to show. I know the answer to the first question but not to the second one.

I've constructed my equations as following

x1 + x2 +x3 + x4 =40

x1 + x2 + x3 = 20

x3 + x3 +X4 = 30

I don't know what to do from there.
 
The system is correct except that the last equation should say $x_2+x_3+x_4=30$ and $x$'s should be all lowercase or all uppercase (preferably, the former).

Do you know about the matrix of a system of equations? Echelon normal form? Rank of a matrix? Rouché–Capelli theorem? Determinants?

Note that by subtracting the second equation from the first one, $x_4$ is uniquely determined. But what about $x_3$? Can you let it equal any number and still find $x_2$ and $x_1$?
 
Yes I do know all that stuff. I was thinking of letting x3 = 10?
 
orgekas said:
I was thinking of letting x3 = 10?
My question was, Can you let $x_3$ equal any number and still find $x_2$ and $x_1$? And the answer is yes. Therefore, the system has infinite number of solutions.
 

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