Why a Negative Times a Negative is a Positive

Question

mathslover · Answer

for example : -(-4) =4 why ?

jackellyn · Answer

http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/negative.html

parthkohli · Answer

I'd give you a fair explanation of this. 
-4 * 1 = -4 because we add -4 to itself 1 time, right?

mathslover · Answer

hmn

parthkohli · Answer

When we add -4 to itself -1 times, the only thing we could do is to do the opposite sign.

parthkohli · Answer

-1 time(s)

parthkohli · Answer

I think you are not satisfied by this explanation. Are you?

mathslover · Answer

http://www.youtube.com/watch?v=rK4sXm_MPWo&feature=em-uploademail good @ParthKohli

parthkohli · Answer

Oh I see, you found Khanacademy. I like his videos very much.

mathslover · Answer

i am in confusion that what software do they use ?

parthkohli · Answer

He uses Camtasia studio, and SmoothDraw 3.
I have both of them :)

parthkohli · Answer

And he uses a graphic tablet. It's from a company called Bamboo, if I am not wrong.

mathslover · Answer

but how did he change that pointer there ?

experimentx · Answer

you can say this interms of inverse
a - a = 0, -a being additive inverse of a
let -a = b
then a+b = 0
then additive inverse of b is -b
or, b - b = 0 => (-a)+(-(-a)) = 0
which says that -(-a) is same as a ... 

inverse are always unique ...

mathslover · Answer

thanks @experimentX  
do u know any thing about that @ParthKohli

parthkohli · Answer

About what?

mathslover · Answer

how to change the pointer ? that khan changed