1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    HallsofIvy

    User Avatar
    Staff Emeritus
    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

    cristo

    User Avatar
    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 :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Why is this matrix undefined?
  1. Matrix ? (Replies: 4)

  2. Proving 0/0 is undefined (Replies: 34)

Loading...