# Homework Help: Finding x and y in matrix

1. May 12, 2008

### Ry122

Can someone tell me what i'm doing wrong. the answer for x is supposed to be -3/7 and y is supposed to be 5/7 but i have 5/7 and -23/7.

$$\begin{bmatrix} 1 & -2 \\ 3 & 1 \end{bmatrix} \begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix}$$
new r1= r1-(r2 x -2)
$$\begin{bmatrix} 7 & 0 \\ 3 & 1 \end{bmatrix} \begin{bmatrix} 1 & -2 \\ 0 & 1 \end{bmatrix}$$
new r1=r1/7
$$\begin{bmatrix} 1 & 0 \\ 3 & 1 \end{bmatrix} \begin{bmatrix} 1/7 & -2/7 \\ 0 & 1 \end{bmatrix}$$
new r2 = r2-(r1 x 3)
$$\begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix} \begin{bmatrix} 1/7 & -2/7 \\ -3/7 & 13/7 \end{bmatrix}$$

$$b=\begin{bmatrix} 1 \\ -2 \end{bmatrix}$$
so
$$\begin{bmatrix} 1/7 & -2/7 \\ -3/7 & 13/7 \end{bmatrix} \begin{bmatrix} 1 \\ -2 \end{bmatrix}$$

$$4/7 + 1/7 = 5/7$$
$$-26/7 + 3/7 = -23/7$$

Last edited: May 12, 2008
2. May 12, 2008

### Defennder

Are you sure y is supposed to be 5/7? I got -5/7. As for your error, check your first step. The matrix on the left hand side should be 1,2 not 1,-2.

3. May 12, 2008

### Ry122

yes its negative 5/7 actually.

4. May 12, 2008

### HallsofIvy

It would help if you would tell us right at the beginning what the problem is!
I can guess, as Defennnder apparently did, that you are trying to solve
$$\begin{bmatrix}1 & -2 \\3 & 1\end{bmatrix}\begin{bmatrix} x \\ y\end{bmatrix}= \begin{bmatrix}1 \\ -2\end{bmatrix}$$
and that you found the inverse matrix then mutliplied the inverse by b.

Here's your error. In the last column "r1- (r2*-2)" would be 0- (1(-2))= 2, not -2. it would have been simpler to think "r1= r1+ (r2*2)" rather than having the two negatives.