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Matrices Help

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  1. Mar 12, 2016 #1
    1. The problem statement, all variables and given/known data
    1hc9pg.jpg
    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. jcsd
  3. Mar 12, 2016 #2

    Merlin3189

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    If you look at Q.2 you have not copied it correctly. Matrix multiplication does not commute.
     
  4. Mar 12, 2016 #3

    HallsofIvy

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    The problem asks you to find the matrix product [tex]\begin{bmatrix}x & 1 \end{bmatrix}\begin{bmatrix} 6 & -3 \\ 4 & 5\end{bmatrix}[/tex]. 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?
     
  5. Mar 12, 2016 #4

    Merlin3189

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    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##
     
  6. Mar 12, 2016 #5

    HallsofIvy

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    Yes, I misread the first line!
     
  7. Mar 13, 2016 #6
    Oh, now I understand! I always have a problem with reading the question properly. Thank you both!
     
  8. Mar 13, 2016 #7

    Merlin3189

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    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.
     
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