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Homework Help: Why is this matrix undefined?

  1. May 4, 2008 #1
    3D + E

    D = 2 x 3

    -1 2 3
    4 0 5

    E = 3 x 2

    2 1
    8 -1
    6 5

    D has 3 columns, and E has 3 rows ???
    Last edited: May 4, 2008
  2. jcsd
  3. May 4, 2008 #2
    Oh crap, ignore me ... LOL, I'm thinking inner product rules ... :p
  4. May 4, 2008 #3


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    Science Advisor

    ??? What matrix are you talking about? You give two matrices, D and E, both of which are defined because you just defined them.

    The product DE is also defined but ED is not. Is that what you are talking about? Do I get a prize for guessing that?

    The product of of two matrices, A and B can be defined as "the ij-component is the dot product of vectors consisting of the ith row of A and the jth column of B".

    ED is not defined because each row of E has 2 components while each column of D has 3 components. You cannot take the dot product of two such vectors.

    As you point out, the number of columns of D and the number of rows of E are the same- that is why DE is defined.
  5. May 4, 2008 #4
    Sorry Ivy! I misread the problem and kept thinking I was multiplying the two, the problem actually asks the addition of the two. I'm not actually doing the problem, just skimming through the section.
  6. May 5, 2008 #5


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    Staff Emeritus
    Science Advisor

    Perhaps less skimming and more 'doing' is in order! :wink:
  7. May 5, 2008 #6
    LOL, I know I should be doing the problems :( But, I did the examples and looked over the rules. I plan on doing a a good review after finals :)
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