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-1*-1=1 ? how to prove

  1. Nov 19, 2007 #1


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    [SOLVED] -1*-1=1 ? how to prove

    1. The problem statement, all variables and given/known data
    Well as the subject states..how does one prove that -1*-1=1?
    I think you have to use proof by contradiction but I dont know what to do after assuming that it is not true.
  2. jcsd
  3. Nov 19, 2007 #2


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    Ok I think I got it...I was told that I should start by knowing that 0*0=0 and that 1-1=0
    such that 0*0=0 => (-1+1)*(-1+1)=(-1*-1)-1-1+1=0
    and so -1*-1=1
  4. Nov 19, 2007 #3
    Well, think about what you know about (-1).

    Then, think about what you know about (0).

    More specifically, note:

    0 = (1 + (-1)) and 0 = ((-1)+1)


    0 = 0*0.

    You should be able to get the algebraic proof from there.

    EDIT: Dang, you got it already. Good job, but don't double post, just edit your original post.
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