Negative times negative is positive?

[Bipolarity]

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]

How rigorous a proof are you looking for?

 

[Bipolarity]

As rigorous as rigorous gets :D

 

[micromass]

As rigorous as rigorous gets :D

What axioms are you accepting??

 

[Dickfore]

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:
<br /> a &lt; 0 \Rightarrow -a &gt; 0<br />
By axiom II.c
<br /> 0 &gt; a \Rightarrow 0 + (-a) &gt; a + (-a) \Leftrightarrow -a &gt; 0<br />
Q.E.D.

Then, look at the following:
<br /> a, b &lt; 0 \Rightarrow a \cdot b = (-a) \cdot (-b)<br />
Then you have a product of two positive numbers, which by the quoted axiom is positive.

 

[Bipolarity]

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

 

[micromass]

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, \mathbb{Z}_2 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