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-1*-1=1 ? how to prove

by rock.freak667
Tags: prove, solved
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Nov19-07, 04:19 PM
HW Helper
P: 6,204
1. The problem statement, all variables and given/known data
Well as the subject 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.
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Nov19-07, 05:22 PM
HW Helper
P: 6,204
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
Nov19-07, 05:36 PM
P: 45
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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