Why is a negative times a negative a positive?

Thanks Xavier - little things like that are great to help people remember things :)

 

Why is a negative times a negative a positive?

 

If you have a debt ot $5 (I mean you owe the bank $5 then you have -5  dollars.

This means that if you get $5 dollars you have to pay it straight to the bank so that will bring your wealth to 0.

-5+5=0

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Now we will start with  - 15 dollars.   That is a 15 dollar debt.

If you take away some of that debt you will have less debt

eg

-15 - (-3) =  dollars worth of debt = -15 - -3 = -15+3 = -12

 

What about if you take away 3 dollars of debt 4 times ?

 

-15 - (4 lots of $3 debt)

=-15 - (4* $3 debt)        hopfully you can see that this is -15- (4*-3) = -15 - - 12 = -15+12 = -3

but another way to do it is

=  -15  - (4* -3)

=  -15  - 4* -3

=  -15   +  (-4*-3)

=  -15   +      +12

=    -3

 

 

I know it is a bit confusing and there are other easier ways to show you using patterns but this should give you something to think about anyway :/

- is 1 because it is a line.

+ is 2 because it gets two.

so - * - = + because it gets two line now.