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N x m matrix value and size

  1. Aug 19, 2010 #1
    1. Suppose P is a 5x4 matrix, Q is an nxm matrix and R is a 3x2 matrix. If (QR)T P is defined then ...

    a) n=4, m=3 or
    b) n=4, m=2

    I have no idea what to do. This is basic stuff... I am doing a correspondence course (no lecturers to help), so I am stuck at this and some of my other questions. Where do I start in solving this? I really want to grasp linear algebra and not just get an answer.

    I know that a 5x4 matrix has 5 rows and 4 columns and that is = P. What do they mean by (QR)T?

    I think that someone who understands this will easily see what they are referring to and unfortunately I am not there at the moment.
  2. jcsd
  3. Aug 19, 2010 #2
  4. Aug 19, 2010 #3
    Hey, thanks for the links...will check it out.
  5. Aug 20, 2010 #4


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    Science Advisor

    In order to multiply AB where A is an m by n matrix and B is a u by v matrix, ("m by n" meaning "m rows, n columns") we must have n= u and the result will be an m by v matrix.

    Since Q is m by n and R is 3 by 2, in order to multiply QR we must have n= 3 and then QR is m by 2 which means its transpose is 2 by m. Now what must m be in order to multiply that by P, a 5 by 4 matrix?
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