Matrices Help

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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

Staff Emeritus
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

Staff Emeritus
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.