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jjc
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I am looking for someone to explain the methodology behind solving simultaneous equations, specifically of four variables. I have read through the numerous other threads about this, and I get most of it, but it always appears that there is some rule or step that I am missing and I am unable to come up with a good answer for my own problem. (This is also my first try at using LaTex. I tried to make it nicely formatted but didn't succeed everywhere. It didn't want to give me new lines inside the Tex tags using the double \. :( )
It really starts out as a request to solve a matrices equation:
Determine w, x, y, z so that this equation holds:
[tex]
\left (\begin{array}{cccc} 2&1&-1&7 \\6&8&0&3 \end{array} \right)
\left (\begin{array}{cc} x & 2x\\ y & -y+z \\ x+w & w-2y+x \\ z & z \end{array} \right)
=
- \left (\begin{array}{cc} 45 & 46\\ 3 & 87 \end{array} \right)
[/tex]
Getting the equations out isn't too difficult. For my own benefit, here is all my work:
A size is 2x4, B size is 4x2; they can by multiplied; resultant array is 2x2 (n=4 for summation)
eq1: [tex] c_{11} = \sum^{n=4}_{j=1} a_{1j}\cdot b_{j1} = 2x + y - x - w + 7z = \underline{x + y - w + 7z} [/tex]
eq2: [tex] c_{12} = \sum^{n=4}_{j=1} a_{1j}\cdot b_{j2} = 2\cdot2x + 1(-y + z) + -1 (w-2y+x) + 7z = \underline{3x + y - w + 8z} [/tex]
eq3: [tex] c_{21} = \sum^{n=4}_{j=1} a_{2j}\cdot b_{j1} = \underline{6x + 8y + 0 + 3z} [/tex]
eq4: [tex] c_{22} = \sum^{n=4}_{j=1} a_{2j}\cdot b_{j2} = 12x -8y + 8z + 0 +3z = \underline{12x - 8y + 11z} [/tex]
And each of the equations above equals the corresponding element from the result matrix.
Now, I have run through these sets 4 or 5 times using different methods (elimination or substitution) but always get answers that don't check. Here was one of my more promising runs:
Add Eq1 and Eq2 (elimination):
Eq1: [tex] x + y - w + 7z = -45 \\ [/tex]
Eq2: [tex] \underline{(-1) (3x + y - w + 8z) = -46 (-1)} \ \ \ \mbox {(multiply by -1 so can add and eliminate)}
} [/tex]
Eq A: [tex] \ \ \ \ -2x + 0 + 0 - z = 1 \ \ \ \ => \ \ \ \underline{z = -1 - 2x}
[/tex]
Now, substitute Eq A into Eq 3:
Eq B: [tex] 6x + 8y + 3(-1 - 2x) = -3 \ \ \ \ => \ \ \ 8y - 3 = -3 \ \ \ \ => \ \ \ 8y = 0 \ \ \ \ => \ \ \underline{y = 0} [/tex]
Now here is where I think things start going wrong. Can I take Eq A and Eq B and throw them back into Eq 4?
Eq C: [tex] 12x - 8\cdot0 + 11 (-1 - 2x) = -87 \ \ \ => \ \ 12x - 11 - 22x = -87 => -10x = -76 \ \ \ \ => \ \ \ \underline { x = 7.6 } [/tex]
OK, not looking too bad there, but I now have two values, and a couple of equations where I can plug values into to find the third: Eq 3, Eq 4, and Eq A. Do I have to throw those values back into original equations and not derived equations (e.g. 3 or 4, but not A)? Can I just put that back into A to find z, then use x, y, and z in any of the other ones to find w?
Eq 3 (with value replacements):
[tex] 6(7.6) + 8(0) + 3z = -3 \ \ \ \ => \ \ \ \ 45.6 + 3z = -3 \ \ \ \ => \ \ \ z = \frac{-3 - 45.6}{3} = \underline{16.2} [/tex]
And lastly, Eq 1 with value replacements:
[tex] 7.6 + 0 - w + 7(16.2) = -45 \ \ \ \ => \ \ \ \underline {w = 76} [/tex]
Hmmmm...this is my cleanest answer yet, and the values for z and w in the last two equations are different than what I have gotten before...
Checking answers:
Eq2 : [tex] 3(7.6) + 0 - 76 + 8(16.2) = -46 \\
22.8 -76 + 129.6 = -46 \ \ \ \ \ \ ? [/tex]
Uh, nope. See! I need to know the background rules so that I can do this correctly and repeatedly instead of haphazardly! And I probably made a number of minor algebraic or arithmetic errors in there, too. :)
Thanks for any pointers!
-- jc
Homework Statement
It really starts out as a request to solve a matrices equation:
Determine w, x, y, z so that this equation holds:
[tex]
\left (\begin{array}{cccc} 2&1&-1&7 \\6&8&0&3 \end{array} \right)
\left (\begin{array}{cc} x & 2x\\ y & -y+z \\ x+w & w-2y+x \\ z & z \end{array} \right)
=
- \left (\begin{array}{cc} 45 & 46\\ 3 & 87 \end{array} \right)
[/tex]
Homework Equations
Getting the equations out isn't too difficult. For my own benefit, here is all my work:
A size is 2x4, B size is 4x2; they can by multiplied; resultant array is 2x2 (n=4 for summation)
eq1: [tex] c_{11} = \sum^{n=4}_{j=1} a_{1j}\cdot b_{j1} = 2x + y - x - w + 7z = \underline{x + y - w + 7z} [/tex]
eq2: [tex] c_{12} = \sum^{n=4}_{j=1} a_{1j}\cdot b_{j2} = 2\cdot2x + 1(-y + z) + -1 (w-2y+x) + 7z = \underline{3x + y - w + 8z} [/tex]
eq3: [tex] c_{21} = \sum^{n=4}_{j=1} a_{2j}\cdot b_{j1} = \underline{6x + 8y + 0 + 3z} [/tex]
eq4: [tex] c_{22} = \sum^{n=4}_{j=1} a_{2j}\cdot b_{j2} = 12x -8y + 8z + 0 +3z = \underline{12x - 8y + 11z} [/tex]
And each of the equations above equals the corresponding element from the result matrix.
The Attempt at a Solution
Now, I have run through these sets 4 or 5 times using different methods (elimination or substitution) but always get answers that don't check. Here was one of my more promising runs:
Add Eq1 and Eq2 (elimination):
Eq1: [tex] x + y - w + 7z = -45 \\ [/tex]
Eq2: [tex] \underline{(-1) (3x + y - w + 8z) = -46 (-1)} \ \ \ \mbox {(multiply by -1 so can add and eliminate)}
} [/tex]
Eq A: [tex] \ \ \ \ -2x + 0 + 0 - z = 1 \ \ \ \ => \ \ \ \underline{z = -1 - 2x}
[/tex]
Now, substitute Eq A into Eq 3:
Eq B: [tex] 6x + 8y + 3(-1 - 2x) = -3 \ \ \ \ => \ \ \ 8y - 3 = -3 \ \ \ \ => \ \ \ 8y = 0 \ \ \ \ => \ \ \underline{y = 0} [/tex]
Now here is where I think things start going wrong. Can I take Eq A and Eq B and throw them back into Eq 4?
Eq C: [tex] 12x - 8\cdot0 + 11 (-1 - 2x) = -87 \ \ \ => \ \ 12x - 11 - 22x = -87 => -10x = -76 \ \ \ \ => \ \ \ \underline { x = 7.6 } [/tex]
OK, not looking too bad there, but I now have two values, and a couple of equations where I can plug values into to find the third: Eq 3, Eq 4, and Eq A. Do I have to throw those values back into original equations and not derived equations (e.g. 3 or 4, but not A)? Can I just put that back into A to find z, then use x, y, and z in any of the other ones to find w?
Eq 3 (with value replacements):
[tex] 6(7.6) + 8(0) + 3z = -3 \ \ \ \ => \ \ \ \ 45.6 + 3z = -3 \ \ \ \ => \ \ \ z = \frac{-3 - 45.6}{3} = \underline{16.2} [/tex]
And lastly, Eq 1 with value replacements:
[tex] 7.6 + 0 - w + 7(16.2) = -45 \ \ \ \ => \ \ \ \underline {w = 76} [/tex]
Hmmmm...this is my cleanest answer yet, and the values for z and w in the last two equations are different than what I have gotten before...
Checking answers:
Eq2 : [tex] 3(7.6) + 0 - 76 + 8(16.2) = -46 \\
22.8 -76 + 129.6 = -46 \ \ \ \ \ \ ? [/tex]
Uh, nope. See! I need to know the background rules so that I can do this correctly and repeatedly instead of haphazardly! And I probably made a number of minor algebraic or arithmetic errors in there, too. :)
Thanks for any pointers!
-- jc
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