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Prove the following euqlaity of determinants

  1. May 24, 2009 #1
    1. The problem statement, all variables and given/known data
    Prove
    det [a+p b+r c+s; d e f; g h i] = det [ a b c; d e f; g h i] + det [p r s; d e f; g h i]



    2. Relevant equations
    none


    3. The attempt at a solution
    i'm not sure how to prove this though its seems obviously true =S
     
  2. jcsd
  3. May 24, 2009 #2

    tiny-tim

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    Hi Jennifer1990! :smile:

    General tip: if you can't see how to prove something, always go back to the definition. :wink:

    Sooo … what is the definition of the determinant? :smile:
     
  4. May 25, 2009 #3
    Re: determinants

    A determinant is a function depending on n that associates a scalar, det(A), to an n×n square matrix A.

    I dont see how this helps me tho.. I'm sorry T_T
     
  5. May 25, 2009 #4

    Mark44

    Staff: Mentor

    Re: determinants

    That's a very general description of what a determinant is, but not a definition. Do you know how to calculate the determinant of a 3 x 3 matrix?
     
  6. May 25, 2009 #5
    Re: determinants

    To calculate the determinant of a 3x3 matrix, u take the sum of the cofactors multiplied by the number in each entry of the original matrix . which is then also multiplied by (-1)^n, where n represents the position of the entry
     
  7. May 25, 2009 #6

    Mark44

    Staff: Mentor

    Re: determinants

    Not quite. As you have written it, it appears that you are saying you have a sum of 9 terms for a 3 x 3 matrix, which is not true.

    You can evaluate a 3 x 3 matrix by cofactors going across any row or down any column. So for your first matrix, one way of evaluating the determinant is to expand across the top row, resulting in (a + p)A11 - (b + r)A12 + (c + s)A13. The Aijs are the cofactors, meaning that each one is the determinant of the 2 x 2 matrix made up of the entries not in row i and column j. I have already taken into account the appropriate sign of each term.

    Now, have at it...
     
  8. May 25, 2009 #7
    Re: determinants

    um actually, thats wat i meant o.o
     
  9. May 25, 2009 #8

    Mark44

    Staff: Mentor

    Re: determinants

    But that's not what you said, so it seemed to me that you didn't really understand how to calculate the determinant.
     
  10. May 25, 2009 #9
    Re: determinants

    ohh sorry.
    so, um, are u asking me to apply that formula to the determinants in the question and show that the determinants are the same?
     
  11. May 25, 2009 #10

    Mark44

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    Re: determinants

    Yes, of course.
     
  12. May 25, 2009 #11

    tiny-tim

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    (just got up :zzz:)

    me too! :biggrin:
     
  13. May 25, 2009 #12
    Re: determinants

    Is there another way to prove this question other than finding the determinant for the matrices?
     
  14. May 25, 2009 #13

    tiny-tim

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    but the question is about the values of the determinants!!!

    so, noooo :rolleyes:

    get on with it! :smile:
     
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