Negative times negative is positive?

  • Thread starter Bipolarity
  • Start date
775
1
Probably the stupidest question I have ever asked, but is it possible to prove that the multiplication of two negatives yields a positive? Go easy on me I've asked better questions :D

BiP
 

Char. Limit

Gold Member
1,198
12
How rigorous a proof are you looking for?
 
775
1
As rigorous as rigorous gets :D
 
2,956
5
These are the usual axioms of real numbers system:

http://www.gap-system.org/~john/analysis/Lectures/L5.html

Specifically, look at the 2nd axiom of order.

EDIT:
Then, prove the following:
[tex]
a < 0 \Rightarrow -a > 0
[/tex]
By axiom II.c
[tex]
0 > a \Rightarrow 0 + (-a) > a + (-a) \Leftrightarrow -a > 0
[/tex]
Q.E.D.

Then, look at the following:
[tex]
a, b < 0 \Rightarrow a \cdot b = (-a) \cdot (-b)
[/tex]
Then you have a product of two positive numbers, which by the quoted axiom is positive.
 
775
1
Ahh, so this is only true for ordered fields then? Is there any number system where the field is not ordered (i.e. does not satisfy axiom II) ?

BiP
 
21,992
3,272
Ahh, so this is only true for ordered fields then? Is there any number system where the field is not ordered (i.e. does not satisfy axiom II) ?

BiP
Sure, [itex]\mathbb{Z}_2[/itex] is not an ordered field.

If you don't know what it is: it's just the set {0,1} with

0+0=1+1=0, 1+0=0+1=1
0*0=0*1=1*0=0, 1*1=1
 

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