Is the System of Equations Inconsistent Despite a Solved Example?

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Discussion Overview

The discussion revolves around a system of equations that a participant believes they have solved, but which is claimed to be inconsistent according to a textbook. The conversation explores the methods of solving the system, particularly using Gauss-Jordan elimination, and addresses the concept of determinants in relation to the consistency of the system.

Discussion Character

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

Main Points Raised

  • One participant presents a system of equations and claims to have found a solution, expressing confusion over the inconsistency stated in the textbook.
  • Another participant asserts that the claimed solution does not satisfy all equations in the system, emphasizing that a valid solution must work for all equations.
  • A different participant suggests a method to manipulate the equations to demonstrate inconsistency, leading to a derived equation that indicates a contradiction.
  • One participant mentions the determinant of the coefficient matrix being zero, implying that this indicates no unique solution exists.
  • Another participant agrees that the determinant being zero suggests inconsistency but notes that this is not always the case.
  • A later reply indicates that the original poster eventually found the system to be inconsistent after further attempts, although they are unsure of their earlier mistakes.
  • Another participant reflects on the commonality of making simple errors in calculations, suggesting that repeated attempts can lead to clarity.

Areas of Agreement / Disagreement

Participants generally agree that the system is inconsistent, but there is some debate regarding the implications of the determinant being zero and whether this always indicates inconsistency.

Contextual Notes

Some participants mention the need for specific arithmetic steps to identify mistakes, indicating that the discussion may depend on individual calculations and interpretations of the equations.

SomeRandomGuy
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0a - 2b + 3c = 1
3a + 6b - 3c = -2
6a + 6b + 3c = 5

I got a = 3, b = 3/2, c = -4/3 but the book says that this system is inconsistent. It asks us to use Gauss-Jordan elimination (which I have used on the past 5 or so problems and got all the right answers). I know how to do these, I just don't see where my mistake is. I am probably looking over some very simplistic reason why it is inconsistent... Any help is appreciated.
 
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The numbers you have found are not the solutions of this system , because they don't give correct results, if put in any of the equations of the system. And a solution of any system must give correct results for all the equations, so if you find a solution (a,b,c) this must be a solution for all the equations.
In your problem you can multiply the second equation with -2 and then add it with the third one(Gauss method). Your system ends up like this:
| 0a-2b+3c=1 | 0a-2b+3c=1
| -6a-12b+6c=4 <=>| -6a-12b+6c=4
| 6a+6b+3c=5 | -6b+9c=9

the last equation can be written like this: -2b+3c=3
So if you add the first one and the {[-2b+3c=3] multiplied with -1} you get 0=-2
This last one is actually this equation : 0a+0b+0c=-2. So the initial system has the same solutions as this system:
0a+0b+0c=-2
-6a-12b+6c=4
-6b+9c=9

obviously this system is inconsistent, so the initial one is also inconsistent as they are equivalent
 
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For us to see your mistake you would have to show us your arithemtic. How many times have you repeated the calculations?
 
I don't know why you struggled so much,the determinant of the coefficients is zero,therefore,no unique solution.

Daniel.
 
The detrminant of the coefficient matrix is equal to zero. Since the matrix is singular, the system is inconsistent.
 
The detrminant of the coefficient matrix is equal to zero. Since the matrix is singular, the system is inconsistent.

Not always.
 
lol thanks guys but I don't know what a determinant is yet (that's chapter 2). I have tried it twice, and have gotten the same result both times...

EDIT: Thanks for your help guys, but on my 4th attempt, I found it to be inconsistent. I don't know what I did wrong the other way because I did it a different way this time. Anyway, thanks for your help.
 
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It's amazing how simple a problem can be the 4th time around.

If I had a dollar for every simple error like that I've made...
 

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