While (-6) + (-6) + (-6) + (-6) = -24
why (-6) * (-4) = 24?
To help him, many people came forward with their examples and proofs. Here is one interesting proof by an assistant professor at NUST. Produced verbatim.
The proof of
-x * -y = x * y
is as follows:
-x * - y = -(x * -y) = --(x * y) = x * y
I have my sympathies with the professor's students. For one, what's this -- symbol? What on earth does that mean? Secondly, what property of arithmetic operators are you using when you convert -x * -y to -(x * -y), and finally isn't the whole argument cyclic? Isn't mathematical rigidity of your proofs the first thing they teach to a PhD candidate? Or have things changed these days?
On a side note, other interesting "logical" posts can be found here:
Jokes aside, the example given of -6 and -4 is plain wrong but the question "why a negative multiplied by a negative number gives a positive number" is a very genuine one. It's very sad that as school children we are not encouraged to ask such questions when a new concept is introduced. But sooner or later one realizes that all of the rules in Mathematics must have a rationale.