Results 1 to 4 of 4

Math Help - Multiplication of Matrices

  1. #1
    Member
    Joined
    Nov 2006
    From
    San Francisco
    Posts
    145

    Multiplication of Matrices

    The question is:

    Let A= [2 5, -3 1] and B=[4 -5, 3 k]

    I don't know if this will represent A and B better:

    [2 , 5] [4 , -5]
    [-3 , 1] [3 , k]

    What value(s) of k, if any, will make AB=BA?


    I got no values of k, but the back of my book claims the answer is (k-5). Can anyone tell me if I am right or wrong?

    Thanks,

    jarny
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Banned
    Joined
    Oct 2009
    Posts
    4,261
    Thanks
    2
    Quote Originally Posted by jarny View Post
    The question is:

    Let A= [2 5, -3 1] and B=[4 -5, 3 k]

    What value(s) of k, if any, will make AB=BA?


    I got no values of k, but the back of my book claims the answer is (k-5). Can anyone tell me if I am right or wrong?

    Thanks,

    jarny

    If you meant to write that your book's answer is k=5 then your book's right.

    Tonio
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    Nov 2006
    From
    San Francisco
    Posts
    145
    I got A*B= [23 (5k-10), -9 k+15]
    and B*A= [23 15, (6-3k) (k+15)]


    Oh okay when I write it on the computer I see I didn't see a negative next to my nine the first time I did it. Thanks tonio
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Banned
    Joined
    Oct 2009
    Posts
    4,261
    Thanks
    2
    Quote Originally Posted by jarny View Post
    I got A*B= [23 (5k-10), -9 k+15]
    and B*A= [23 15, (6-3k) (k+15)]


    Oh okay when I write it on the computer I see I didn't see a negative next to my nine the first time I did it. Thanks tonio

    Fine, but a piece of advise: do not do mathematics with your computer! Grab a pencil (or a pen), a sheet of paper (or better: 37436 sheets of paper) and begin to write. Much better if you can get chalk and a blackboard.
    After you do maths this way, then you can go to the puter and write it down nicely.

    Tonio
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Multiplication of Matrices
    Posted in the Algebra Forum
    Replies: 1
    Last Post: April 8th 2010, 08:36 PM
  2. multiplication of matrices
    Posted in the Advanced Algebra Forum
    Replies: 2
    Last Post: January 26th 2009, 09:03 PM
  3. Matrices Multiplication
    Posted in the Algebra Forum
    Replies: 6
    Last Post: January 15th 2009, 03:07 AM
  4. [SOLVED] Multiplication of matrices
    Posted in the Advanced Algebra Forum
    Replies: 4
    Last Post: September 15th 2008, 10:02 AM
  5. Matrices and multiplication commutativity
    Posted in the Advanced Algebra Forum
    Replies: 2
    Last Post: June 24th 2008, 11:33 PM

Search Tags


/mathhelpforum @mathhelpforum