# Homework Help: Matrices Help

Tags:
1. Mar 12, 2016

### zeldaspurpose

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
I need help with the second question, I did the first one correctly. My pre board is on Monday so please help.

2. Mar 12, 2016

### Merlin3189

If you look at Q.2 you have not copied it correctly. Matrix multiplication does not commute.

3. Mar 12, 2016

### HallsofIvy

The problem asks you to find the matrix product $$\begin{bmatrix}x & 1 \end{bmatrix}\begin{bmatrix} 6 & -3 \\ 4 & 5\end{bmatrix}$$. I have no idea what you found! You used the answer to the previous problem, MX, rather than M, and somehow multiplied x only by the "6x" in "6x- 3" rather than both?

4. Mar 12, 2016

### Merlin3189

I don't agree with Hallsoflvy.
$M = \begin{bmatrix} 6 & -3 \\ 4 & 5\end{bmatrix}\ \ \begin{bmatrix}x \\ 1 \end{bmatrix} \ \ =\ \begin{bmatrix} (6x-3) \\ (4x+5)\end{bmatrix}$ as OP said.

$M \neq \begin{bmatrix} 6 & -3 \\ 4 & 5\end{bmatrix}$ as Hallsoflvy claims.

Therefore in part b, you are asked to find
$\begin{bmatrix}x & 1 \end{bmatrix} M \ \ =\ \begin{bmatrix}x & 1 \end{bmatrix}\ \begin{bmatrix} (6x-3) \\ (4x+5)\end{bmatrix}$
which is what neither said.
OP's error was to write $M\ \begin{bmatrix}x & 1 \end{bmatrix}$ instead of $\begin{bmatrix}x & 1 \end{bmatrix}\ M$

5. Mar 12, 2016

### HallsofIvy

Yes, I misread the first line!

6. Mar 13, 2016

### zeldaspurpose

Oh, now I understand! I always have a problem with reading the question properly. Thank you both!

7. Mar 13, 2016

### Merlin3189

Just remember, with matrices order is important.

And congratulations on asking the question clearly with a nice uploaded pic. Some questioners go on for a dozen posts or more before you find out what they actually need.