# Matrix question

1. Sep 20, 2005

Use the Gauss-Jordan algorithm to find all solutions of the following system of linear equations in C:

x1 + x2 + x3 [] = 2
2x1 [] + 2x3 + 2x4 = 2
x1 + 2x2 + 2x3 [] = 1
2x1 + 2x2 [] + x4 = 2

[] signify a blank space in the equation. How do you even proceed to do this, I have never seen it. Any help would be appreciated. Thank you.

2. Sep 20, 2005

### TD

In matrix-form, it looks like this:

$$\left( {\begin{array}{*{20}c} 1 & 1 & 1 & 0 & 2 \\ 2 & 0 & 2 & 2 & 2 \\ 1 & 2 & 2 & 0 & 1 \\ 2 & 2 & 0 & 1 & 2 \\ \end{array}} \right)$$

Do you know how Gaussian elimination works?

3. Sep 20, 2005

### saltydog

$$\left( \begin{array}{cccc|c}1 & 1 & 1 & 0 & 2\\ 2 & 0 & 2 & 2 & 2 \\ 1 & 2 & 2 & 0 & 2 \\ 2 & 2 & 0 & 1 & 1 \end{array}\right)$$

So, reduce it. Here, I'll do the first part. I'll multiply the top row by -2 and then add it to the second row yielding:

$$\left( \begin{array}{cccc|c}1 & 1 & 1 & 0 & 2 \\ 0 & -2 & 0 & 2 & -2 \\ 1 & 2 & 2 & 0 & 2 \\ 2 & 2 & 0 & 1 & 1 \end{array}\right)$$

Can you continue doing this until you get it to row-reduced form?

Last edited: Sep 21, 2005
4. Sep 20, 2005

### HallsofIvy

Typo!! The first matrix in Saltydog's response should be
$$\left(\begin{array}{cccc|c}1 & 1 & 1 & 0 & 2\\ 2 & 0 & 2 & 2 & 2 \\ 1 & 2 & 2 & 0 & 1 \\ 2 & 2 & 0 & 1 & 2 \end{array}\right)$$

Except for separating out the "augmenting" part, that's just what TD said.

5. Sep 20, 2005

Hmm..looks simple enough, just did not know what to do with those blank spots...Ok, I got this,

$$\left( \begin{array}{cccc|c}1 & 0 & 0 & 0 & 3\\ 0 & 1 & 0 & 0 & -1 \\ 0 & 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 1 & -2 \end{array}\right)$$

so um,
x_4 = -2x_4
x_3 = 0
x_2 = -x_2
x_1 = 3x_1

are those the solutions?
Thank you for the help thus far...

6. Sep 21, 2005

### TD

I think you row-reduced fine but the last column refers to the constants, not to an unknown. So the solution should be:

$$\left\{ \begin{array}{l} x_1 = 3 \\ x_2 = - 1 \\ x_3 = 0 \\ x_4 = - 2 \\ \end{array} \right$$