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Homework Help: Matrix Multipication

  1. Mar 28, 2010 #1
    1. The problem statement, all variables and given/known data

    I look at the following:

    [tex](x_1\,\,x_2)\begin{pmatrix}1&2\\2&4\end{pmatrix}\binom {x_1}{x_2}=(x_1\,\,x_2)\binom {x_1+2x_2}{2x_1+4x_2}=x_1^2+2x_1x_2+2x_1x_2+4x_2^2[/tex]

    And I don't understand how they got from [tex](x_1\,\,x_2)\binom {x_1+2x_2}{2x_1+4x_2}[/tex] to [tex]x_1^2+2x_1x_2+2x_1x_2+4x_2^2[/tex].

    3. The attempt at a solution

    What process is used here? It surely is not matrix multiplication, otherwise the result would have to be a 1x2 matrix. :confused:
  2. jcsd
  3. Mar 28, 2010 #2


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    Hi roam! :smile:

    No, that longy thingy is a 1x2 column. :wink:
  4. Mar 28, 2010 #3
    Concerning matrix multiplication, the rule is that a (mxn) matrix multiplied by a (nxp) matrix gives a (mxp) matrix.
    Here, you have a (1x2) matrix multiplied by a (2x2) multiplied by a (2x1). Which indeed leeds to a (1x1) matrix i.e. a number.
    The part you don't understand is a (1x2) x (2x1) and applying the rules of matrix multiplication you indeed find the result :smile:
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