# Simultaneous equations Matrices

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#1
Hey guys, really confused with this. Basically using matrices to work out simultaneous equations.
I know how to do it, but I often get confused with the first step, that is rearranging the equation.

For example:

3X1 + X2 = -7
X2 -2X1 = -2

The bottom equation must be rearranged, to conduct with the matrix. So it becomes:
2x1-X2 = +2
Note how -2 becomes +2

But in a different example, rearrangement is completely different
x1 - x2 = -3
x2-2x1 = 4
becomes
-2x1 + X2 = 4
note here, 4 stays as +4 and the equation is rearranged differently

why is this?

Thanks
0
6 years ago
#2
The first time, they've multiplied through by -1, so the coefficient of X_1 is positive (looks neater). The second time, this isn't necessary, they've just moved X_1 in front of X_2..
1
#3
(Original post by Jooooshy)
The first time, they've multiplied through by -1, so the coefficient of X_1 is positive (looks neater). The second time, this isn't necessary, they've just moved X_1 in front of X_2..
Thanks for the help.
3X2 - 2X1 = -12

Mark scheme says it becomes:
-2X1 + 3X2 = -12

I would've thought it becomes: 2X1 - 3X2 = +12

Thing is, I get the right answer even when I multiply by -1, but my working out is completely different to the Mark scheme, as step 1 (rearranging) is completely different.
0
6 years ago
#4
(Original post by 16dan2life)
Thanks for the help.
3X2 - 2X1 = -12

Mark scheme says it becomes:
-2X1 + 3X2 = -12

I would've thought it becomes: 2X1 - 3X2 = +12

Thing is, I get the right answer even when I multiply by -1, but my working out is completely different to the Mark scheme, as step 1 (rearranging) is completely different.
It doesn't make a difference!

All they've done is move the terms around so the X1 term comes first. You've done this AND multiplied both sides by -1. Both ways lead to the correct answer
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