Question

Why (-)*(-)=(+).

Answer 1

hmmm.......

multiplication is adding really

if you add the opposite of 3 that opposite of 5 times we get what?

Answer 2

the opposite of 3; 5 times =

-3 -3 -3 -3 -3 = -15

so the opposite of 5 times gets you to 15

Answer 3

-(-3)-(-3)-(-3)-(-3)-(-3) = 15

Answer 4

does that make sense?

Answer 5

it takes me to other sense that is (-1)*x=1 then x=(-1)

Answer 6

Do you understand / accept (-1)*(1) = -1 ?

Answer 7

yeah

Answer 8

So then you get \[0 = 0\cdot 0 = (a-a)\cdot(b-b) = (a+(-a))\cdot(b+(-b))\]so using distributivity\[ = a\cdot b + a\cdot (-b) + (-a)\cdot b + (-a)\cdot(-b) = a\cdot b - a\cdot b - a\cdot b + (-a)\cdot (-b)\]thus altogether \[ 0= (-a)\cdot(-b) - a\cdot b\] and that means nothing else than \[(-a)\cdot(-b) = a\cdot b\]

Answer 9

phhhhbbbtttt!!! how needs proofs ;)

Answer 10

hehe, Mathematics needs it, or from a certain point on anything will be arbitrary because you can't tell intuitively what's true and what's false.

Answer 11

btw 555 medals, awesome work there :-)