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Homework Help: Multiplication of an Identity Matrix by a Column

  1. Oct 30, 2014 #1
    1. The problem statement, all variables and given/known data

    This is a seemingly simple problem. All I have to do is multiply two matrices:

    [ 1 0 ]
    [ 0 1 ] (A)


    [ 2 ]
    [ 3 ] (B)

    3. The attempt at a solution

    Because the matrix A has the same number of columns as matrix B has rows, and because matrix A is an identity matrix, I would expect the answer to just be matrix B. But the back of the book says the answer does not exist which boggles me. Can anyone share any useful insight?
  2. jcsd
  3. Oct 30, 2014 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    It's not clear from your problem statement whether you are supposed to calculate AB or BA. They're not the same calculation.
  4. Oct 30, 2014 #3


    Staff: Mentor

    Where is your problem statement? What is A? What is B? I can see you have the two-dimension identity matrix in your first product, but what is the significance of the vector <2, 3> in the second product? Without knowing anything about A and B and without a clear statement of the problem it's impossible to provide you with any help.
  5. Oct 30, 2014 #4


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    Homework Helper

    If you have a 2x2 times a 2x1, this is okay and will yield a 2x1 matrix.
    If you have a 2x1 times a 2x2, this does not work. The # of columns in your first matrix must equal the # of rows in your second. The 2x2 identity matrix can be right or left multiplied onto any other 2x2 matrices with the expected result, but other than that, you have to match your dimensions.
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