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July 07, 2026, 02:23:54 am

Author Topic: simultaneous equations?  (Read 1293 times)  Share 

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wombifat

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simultaneous equations?
« on: April 12, 2009, 03:25:26 pm »
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Find the value of a for which there are infinitely many solutions to the equations

2x+ay-z=0
3x+4y-(a+1)z=13
10x+8y+(a-4)z=26

As a matter of fact, can anybody explain to me how to work out when an equation has infinitely many solution\no solutions and all that, or point me to somewhere that explains it, but it isn't explained in the book and I don't think we went over it very thoroughly in class.

Mao

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Re: simultaneous equations?
« Reply #1 on: April 12, 2009, 05:07:44 pm »
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Looking at the right hand sides, you have 0, 13 and 26. In order to have infinite solutions, one equation must be a sum of multiples of other equations (i.e. linearly dependant).

That is,

Hence, using the same relationship for each coefficients:





checking using the final column:



Hence,

This question is most likely out of the course though.
« Last Edit: April 12, 2009, 05:10:15 pm by Mao »
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wombifat

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Re: simultaneous equations?
« Reply #2 on: April 12, 2009, 05:10:21 pm »
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seriously? It was in the text book. Well thanks heaps I've been refreshing the page till I can get an answer.

TrueTears

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Re: simultaneous equations?
« Reply #3 on: April 12, 2009, 05:11:30 pm »
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Yes, our teacher said these kind of questions are not in the methods course.
PhD @ MIT (Economics).

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Mao

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Re: simultaneous equations?
« Reply #4 on: April 12, 2009, 05:49:07 pm »
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I'm guessing you are using the essentials textbook. They have quite a few matrices questions that aren't actually in the course.

To answer your more general question about infinite and no solutions (I will not explain why, but will explain how to find them, because knowing 'why' is beyond 'methods' :) ) :

For infinite solutions, the equations are 'linearly dependant', that is, , where are real constants and eq1, eq2 and etc are equations.
That is, each set of coefficients (i.e. coefficients of x, coefficients of y, coefficients of z, constant term on RHS) must all follow this pattern. Solving this (see post above) will give you infinite solutions.

For singular solutions, use the rref function. For the above case, do
The result will yield
Where x=k, y=m, z=n (for a given value of a, which gets substituted into the expression for k, m and n)

For no solutions, you try to find values of the variable such that k, m and n do not exist. In this case, k, m and n all have a common denominator of , that is, if a=2, then you will be dividing by zero undefined. Hence if a=2, there are no solutions.
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wombifat

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Re: simultaneous equations?
« Reply #5 on: April 12, 2009, 06:07:24 pm »
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ok thanks, I don't think I fully understood that but it definitely helps :)