# How can you multiply two negative numbers?

#### Dark Fire

TeX doesn't calculate anything; it's just a type setting program. All it does is let you type up nice papers (though it is very useful for writing up your math homework in college if you're taking some advanced courses).
Ahh, never mind, it's a bug in the editing...
I first wrote my whole math, but then it looked all crazy, and it looked like it were calculating every part I wrote, so I edited it to only "-9^-1" and it looked like it were calculating the whole thing, until I refreshed the website...

And btw, I was taught that ^-1 means squareroot.
81^-2 = 3 because, 3^2 = 81

Here's my math:
I=-9^-1
I^2=(-9^-1)^2=81^-2=3

And if ^-1 doesn't mean sqrt then:
I=sqrt-9
I^2=sqrt(-9)^2=cuberoot(81)=3

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#### LukeD

No, x^(-y) = 1/(x^y)

So (-9)^(-1) = 1/((-9)^1) = 1/(-9) = -1/9

However, x^(1/2) is the square root of x

You might want to look up how exponents work

(also 3^2 = 9, not 81)

#### Trekky0623

So let's assume we have a positive whole number, n.

-n = -1 * n

So let's just say n is 3.

(-1) + (-1) + (-1) = -3

When multiply by a negative, instead of addition, we use subtraction, so:

(-1) * (-3) = -(-1) - (-1) - (-1)

So (-1) * (-3) = 3

#### d_leet

The cube root of 81 is also not equal to 3. 33=27.

#### LukeD

The cube root of 81 is also not equal to 3. 33=27.
Hey, it's your $$2^{10}$$th post!

DarkFire: (81)^(1/4) = 3 and 3^4 = 81. Like I said, you may want to look up how exponents work. I think you're a little confused about them.

Also, one thing that you should know is that for non integer values of y, x^y is not a function. There are for instance two different values for $$x^{1/2}$$. If we let z denote one of the numbers, then -z is the other (it's just then when x is real and positive, we usually take it to be the positive number and when x is negative, we take it to be the "positive" imaginary number). In particular, if n is an integer, then $$x^{1/n}$$ could be 1 of n different numbers. When y is irrational, there is an infinite number of things that $$x^y$$ can be.

So this means that canceling exponentials by taking them to the 1/n power is not always valid. You need to be careful with this or you'll end up with nonsense like 1 = -1

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#### maze

@ the original poster

You can also rephrase the question from the point of view of the distributive law,
1 = 1*1 = (-1+2)*(-1+2) = -1*-1 + -1*2 + -1*2 + 4 = -1*-1

Then you only need to justify the distributive law for negative numbers, which may be easier to think up real-world examples for (?)

#### Peeter

I blundered on the website of the url below, and searched on Euler just for fun and happened to find the following:

"An introduction to the elements of algebra, designed for the use of those who are acquainted only with the first principles of arithmetic. Selected from the algebra of Euler (1818)"

http://www.archive.org/details/introductiontoel00eulerich

His justification for this rule (paraphrasing) is that -a * -b, for positive a & b should have a different sign than a * -b which is negative, thus positive (exact text available above).

#### maze

Euler's argument is not particularly convincing IMO. The counterargument being, that for a negative times a negative, we might want to define it to be a whole other sort of number altogether (eg: -a*-b = ja*b, where j is -1*-1, analogous to i in complex numbers).

#### Gib Z

Homework Helper
It may be simplest if we explain it as if numbers were vectors along the real number line.

For all numbers, we can split it into two things: Its magnitude, and its sign. We can interpret the sign as to be the direction it in going in, and magnitude is how far it goes. A positive sign just means go towards the right, and negative signs just mean switch directions. So having multiplying two negative signs can be seen as changing your direction but then changing back!.

#### Peeter

Euler's argument is not particularly convincing IMO. The counterargument being, that for a negative times a negative, we might want to define it to be a whole other sort of number altogether (eg: -a*-b = ja*b, where j is -1*-1, analogous to i in complex numbers).
His argument works nicely for the i of complex numbers too. Either way it flips the sign, and application twice gets you right back to where you were. This is why I like the geometrical view of negation ... it's a 180 degree rotation, or inversion, depending on the dimension of the space.

#### Borek

Mentor
In terms of quaternions Eulers argument seems weak, I am with maze on that one.

#### Peeter

In terms of quaternions Eulers argument seems weak, I am with maze on that one.
I don't follow what you are getting at? You've got:

i(-j) = -k

so, in my mind his argument would say that one should have:

(-i)(-j) = k

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#### arildno

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Gold Member
Dearly Missed
It's the only way I've been able to understand multiplication, we were also taught to think of it that way when we were childs, and I never had a flash/bright idea/philosophy popping up, telling me maybe multiplication isn't that way of thinking after all: until now.

Thank you all for the replies, feeling good for not being a fool after all :)
It's better to think of multiplication as one particular map (amongst numerous others) between two numbers yielding a third number.

That mapping, called multiplication has numerous "properties" set down as so-called axioms.

To give you an idea about what I mean with "map", we will look at the "addition map A", associating to two numbers a,b a third number c called their "sum".

For example A(1,1)=2 (i.e we associate a=1 and b=1 with the number c=2, calling c the sum of a and b)

Similarly, we will have, for example, A(5,1)=6 A(7,13)=20 and so on.

Such a map has a very intuitive representation as a table.

Your multiplication table, with row numbers and column numbers is precisely such a map, or rather, a PART of such a map.

Thus, rather than thinkinking of multiplying two numbers as meaning "having so and so much" of a quantity, you should rather think of multiplication as a particular set of properties with the multiplication table, those properties enabling you to calculate the product, either by looking up in a pre-written table, or perform the multiplication by "multiplying", i.e, assigning the right c-number to the chosen a,b-numbers.

#### maze

But that is dodging the question. We can all make up number systems with all sorts of properties (or maps, or sets with operations, or however you want to describe it). In fact, people already have done so, and the results are often interesting.

However, what takes math beyond being a random game played with symbols is the strange coincidence that numbers, sets, and so forth actually have a connection to reality (or at least connections to things with a connection to reality). That is what the origional poster is getting at - what is the connection between the formalism and real world things (like paying for a $10 pizza split 5 ways). #### Borek Mentor I don't follow what you are getting at? You've got: i(-j) = -k so, in my mind his argument would say that one should have: (-i)(-j) = k What I meant was that we can extend the idea of sign. We can assume (-1)(-1) = 1, we can assume i^2 = -1, or (-i)(-j) = k - whatever. We can also assume that each combination like -+-++ is a completely new entity. Eulers argument will be wrong then, as it works only when the final result has to have one of two possible signs. In a way Eulers argument is what I hinted at at the very beginning of this thread. #### ehj Well.. let's say that two negatives multiplied gave another negative: (-1)(-1) = -1 and at the same time 1*(-1) = -1 That would imply that (-1)(-1) = 1*(-1) <=> 1 = -1 which is obviously not true. It probably is a convention, although a convention that makes sense as we've just seen! #### Crosson But that is dodging the question. We can all make up number systems with all sorts of properties (or maps, or sets with operations, or however you want to describe it). In fact, people already have done so, and the results are often interesting. However, what takes math beyond being a random game played with symbols is the strange coincidence that numbers, sets, and so forth actually have a connection to reality (or at least connections to things with a connection to reality). That is what the origional poster is getting at - what is the connection between the formalism and real world things (like paying for a$10 pizza split 5 ways).
I think you can't see the purpose of real numbers in an isolated problem, for example about mortgage debt, since even semi-intelligent human calculators can semi-consciously carry their negative signs through a word problem --- e.g. "I never add a positive to a negative, but rather I treat this as a subtraction of two positives in the order of larger - smaller and assign the the parity of the sum to be that of the larger number." or "obviously not paying a $500 expected mortgage for 5 months is not a savings of (-$500)(-5) but rather to (\$500)(5); although these are the same, certainly only a fool with the mind of a machine would actually need to think in terms of the former."

In other words, negative signs only make sense inside of an established system. Consider the formula for average velocity of an object along a line. If we want there to be such a universal equation, we need a convention: positions along the line are represented by real numbers with the particle considered to be at position x = 0 at time t = 0 and the positive axis in the direction along the line most angularly near to north.

This means that the average velocity during the a trip of duration $t$ to position x is given by the equation:

v = x/t

Where negative velocities correspond to negative net displacements. The bottom line is that instead of doing the book keeping in our heads, we can establish the conventions and express ourselves with more brevity. This is not apparent in the system I used as an example, where the weight of the conventions exceeded the value of the "universal" formula I gave, but that is because I wanted it to make the point while still being accessible to everyone.

Furthermore, your critique so far has only been half an argument, since you have made the unconcious assumption that numbers represent either magnitudes, cardinals, or ordinals. In fact, neither the negative reals nor the negative integers are any of these, but rather they are inverses of addition. If that is not real to you, than you are drawing your own arbitrary line in the sand without explaining and defending what you think it means to "connect to real world things."

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#### arildno

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Gold Member
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coincidence that numbers, sets, and so forth actually have a connection to reality (or at least connections to things with a connection to reality).
Nope. That is an attribute of the real world, not of maths.

Maths IS, essentially, games of our own making.

#### CRGreathouse

Homework Helper
Complex numbers allow any polynomial in real (or complex) numbers to be solved. In the same way, (possibly negative) integers allow any monic linear equation to be solved, and rational numbers allow any linear equation to be solved.

Without negatives it's hard to factor polynomials.

#### Borek

Mentor
Nope. That is an attribute of the real world, not of maths.

Maths IS, essentially, games of our own making.
My take is that world have no choice, but to follow math. And while we can treat math as a "game of our own making" math in fact does exist on its own. Just because we are sometimes surprised by the fact that some esoteric math theory describes the way umpth dimension curls in the presence of blablah field proves math existence. After all, world worked the way it does long before we learnt how to count to three.

#### LukeD

Just because we are sometimes surprised by the fact that some esoteric math theory describes the way umpth dimension curls in the presence of blablah field proves math existence.
You're confusing theory with fact. No one knows that any of our physics describes things how they actually are. In fact, most people suspect that every piece of physics that we have to date is actually completely wrong (though correct to some approximation)

#### Borek

Mentor
In fact, most people suspect that every piece of physics that we have to date is actually completely wrong (though correct to some approximation)
I am in minority (which is most likely the majority) then. Still, even if that's true there is no reason for the real physics to not follow math. Experience so far tells us that that's the way it is.

#### LukeD

I am in minority (which is most likely the majority) then. Still, even if that's true there is no reason for the real physics to not follow math. Experience so far tells us that that's the way it is.
Sure, it's possible that the "ultimate laws of physics" are describable by mathematics. But how does that make the math that describes them real? For any phenomenon, there is an infinite number of mathematical frameworks that describe it. If some framework that describes reality uses mathematical concept A, how does that make A "real"?

You're throwing around works like "real" and "mathematics" (not to mention "real physics"), but I have no idea what you even mean by those words, so I can't actually discuss them (and these are by no means words that have well accepted definitions).

Anyway, this conversation is off topic and should probably be moved elsewhere if you wish it to continue (there is a philosophy board which I suspect would be better suited for this)

#### Diffy

most people suspect that every piece of physics that we have to date is actually completely wrong (though correct to some approximation)
Is this true? Do you have a source or is this an opinion?

So if we take the mass of an object, and say said object is accelerating at some rate and we multiply the mass and the acceleration, we would not have the force? Is this due to an error in formula (perhaps we should subtract?), or numerical inaccuracy in measuring mass and acceleration?

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