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Math Proof

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Arkin
Sun Feb 06 2011, 09:30PM Print
Arkin Registered Member #2140 Joined: Tue May 26 2009, 09:16PM
Location:
Posts: 53
I know it isn't exactly electronic-y, but figured I would ask here, where there's a lot of smart people.

I have an assignment, and it is finding patterns within linear equations. The linear equation's coeffecience have a common difference. e.g:

x+2y=3 (difference of 1)
2x-y=-4 (difference of 2)

The pattern here is for any 2x2 equation, with a common difference, the solution is always (-1,2). For a 3x3, it is y=-2x-1, z=x+2.

The problem I have is extending it. This is what I noticed, for the y values:

3x3: -2x-1
4x4: u-2x-2
5x5: 2f+u-2x-3

Where the variable progression is x,y,z,u,f.

The pattern is that everytime, the last (n-3) variable - 1 is added. E.g, -2x-1 is a 3x3. To go to 4x4, the coefficient would be 1 (4-3), so I add u-1. 4x4 to 5x5 is a coefficient of 2 (5-3), so I add 2f-1.

Because this is so arbitrary, with out even a concrete number of terms, how would you go about proving it? My guess is somehow, a bunch of middle terms cancel out, so you only have to worry about the first and last equation.
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Dr. ISOTOP
Sun Feb 06 2011, 10:00PM
Dr. ISOTOP Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
What do you mean "coefficients have a common difference?"
Do you mean that if we write the system as a matrix {a_ij}, the rows of the matrix from arithmetic sequences?
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Arkin
Sun Feb 06 2011, 10:10PM
Arkin Registered Member #2140 Joined: Tue May 26 2009, 09:16PM
Location:
Posts: 53
yes. For a 3x3 system of equations, it would be generalized like this:

ax + (a+n)y + (a+2n)z = a+3n
bx + (b+m)y + (b+2m)z = b+3m
cx + (c+u)y + (c+2u)z = c+3u
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Arkin
Mon Feb 07 2011, 03:35AM
Arkin Registered Member #2140 Joined: Tue May 26 2009, 09:16PM
Location:
Posts: 53
Here is another way of saying it:
F4i

[Edit: Image size]
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woxxey
Mon Feb 07 2011, 03:13PM
woxxey Registered Member #1906 Joined: Sat Jan 03 2009, 05:06PM
Location:
Posts: 39
I think Iv'e done that assignment :P
I only extended it to a 3x3 system and proved it generally using matrices and row operations.
59 Pm

Edit: It might be wort acknowledging that the coefficients form an arithmetic sequence in Part A.
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Arkin
Tue Feb 08 2011, 01:15AM
Arkin Registered Member #2140 Joined: Tue May 26 2009, 09:16PM
Location:
Posts: 53
Yea, chances are you did, as IB as only a hand full =p

I already proved the 3x3, but through substitution, not row operations. I'le have to look up matrix row operations, never done much with matrixes.
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