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Homework Help: Determine an unknown matrix

  1. Feb 12, 2008 #1
    General question:
    How do you determine a matrix B when given A and AB?
  2. jcsd
  3. Feb 12, 2008 #2
    If you know the inverse of A, you can multiply on the left side by A^-1 to get B, i.e. (A^-1)AB = B.
  4. Feb 12, 2008 #3
    Oh I see. I understand the computation, but could you explain the theory behind this?
  5. Feb 12, 2008 #4
    This isn't exactly a thorough explanation, but basically if you multiply a matrix by its inverse, you get the identity matrix I. If you multiply any matrix by the identity matrix, you get the original matrix back again, e.g. IA = AI = A.
  6. Feb 13, 2008 #5


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    Science Advisor

    The theory? A guess it's things like "associative law" and "existence of the multiplicative identity"!
    If you were given two numbers a, c, a not equal to 0, and told that ab= c, how would you solve for b?

    If A has an inverse, then A-1(AB)= (A-1A)B= IB= B.

    Notice the condition "If A has an inverse". There are plenty of different matrices, A, B, not having inverses, such that AB= 0. If A does not have an inverse, there may be many different matrices B such that AB= C.
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