why does (-5)*(-4)=20 ?, eom

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[Cato]

Please tell me you are not from the U.S.

[Captain Colostomy]

Godidit.

Additional commentary- Why am I not surprised that an avowed christian has trouble grasping simple mathematical concepts?

Yes...it's rhetorical.

[Minimalist]

Why am I not surprised that an avowed christian has trouble grasping simple mathematical concepts?

Because they think 1+1+1 = 1?

Just a guess.

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Why am I not surprised that an avowed christian has trouble grasping simple mathematical concepts?

Because they think 1+1+1 = 1?

Just a guess.

guess the answer is so obvious, it can't be explained by the more intelligent?

[Faith No More]

guess the answer is so obvious, it can't be explained by the more intelligent?

Perhaps you could elaborate a little more on where you are having trouble with the problem.

[Aractus]

Perhaps some of you idiots should try explaining if you really do know the reason that two negatives multiplied together give a positive.

I'll give you my take on it - mathematics is invented by humans, and is based on rules, which are also invented. Numbers are not real, and while positive numbers can be likened to reality, somewhat, negative numbers of course cannot. For instance you can 0 Apples, you can 12 Apples, but you can not have -1 Apples. The reason we have negative numbers is that that we can perform negative operations on positive numbers, for instance if you have 12 Apples and negative 9 Apples you now have 3 Apples. Note that you cannot physically have 12 Apples and negative 13 Apples, however you can do this in mathematics. Each integer and each rational number has a negative inverse, which when added together equals zero. For instance 5 + (-5) = 0.

Anyway, the rules are this:

1 * -1 = -1

Now divide each side by -1 (you can just remove it from the left side):

1 = -1/-1

So two negatives divided by each other equal a positive, and the same must be true for multiplication because you can rewrite it like this:

1 = (1/-1) * (-1/1)
1 = -(1/1) * -(1/1)
1 = -1 * -1

That's one reason, and the only other reason I can think of, and it may be the more correct reason, is that -1*-1 = +1 is the only way to solve the equation without causing other mathematical problems and issues with negative numbers. For instance, if -1 * -1 = -1 and +1 * +1 = +1, then how do you transform a negative number into a positive? You can't, but you can transform a positive number into a negative like this: +1 * -1 = -1, so it would have no inverse there would be no way in which to undo it. Another rule that would break is this one - any number "k" divided by itself equals 1 ie k/k = 1, well if -1 * -1 = -1 then for k= -1: -1 / -1 = -1, rule broken!

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guess the answer is so obvious, it can't be explained by the more intelligent?

Perhaps you could elaborate a little more on where you are having trouble with the problem.

two negatives result in a positive (a math rule), got that.
understand it's the right answer, and understand adding two negatives is more negatives. but what's the rationale for two negatives mult. equaling a positive number.

[pocaracas]

Aractus nailed it.
Any number divided by itself must equal 1.
The multiplication rule then comes out of that.

[Faith No More]

Perhaps some of you idiots should try explaining if you really do know the reason that two negatives multiplied together give a positive.

Perhaps he/she should have phrased the question that way, then, because the answer to why -5 * -4 = 20 is that 4 * 5 = 20 and when multiplying two negative numbers, you get a positive. If the last part was what he/she was really after, it should have been phrased more clearly. Apparently, I don't have the mind reading powers that you do.