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Negative × Negative = Positive in 5 Levels -- Elementary to Math Major
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- Опубліковано 15 сер 2024
- Why is negative times negative equal to positive? Let's answer this question in 5 levels, ranging from intuitive examples based on our daily experiences to a proof in Ring Theory that is often only seen by math majors.
Level 0: 4chan
>turn around
>turn around again
>wtf im facing the same direction
real
>don't turn around
>don't turn around again
>wtf i'm facing the same direction
>be me
@@dedede5586 That's positive times positive.
Turn 90°.
Turn 90° again.
Wtf, I'm facing backwards from how I was originally!
And that is why i² = -1.
I used to think of it like this: negative is "not", positive is "yes". Negative times positive is "not yes", which is "no" which is negative. And for negative times negative, it's "not no" which is yes, which is positive.
exactly how i think of it
i use turn around and dont turn around
@@djfghjif that also works
So negative x negative = F in English.
So negative x negative = F in English.
In Vietnamese maths textbooks for grade 6, the intuitive explanation goes like this:
(-5) ×3 = (-5) + (-5) + (5) = -15
(-5) ×2 = (-5) +(-5) = -10
(-5) ×1 = (-5)
(-5) ×0 = 0
There are clear arrows to indicate that as the 2nd factor decreases by 1, the product goes up by 5. And then students are asked to guess what happens when the factor keeps decreasing to negative values. I think it's intuitive enough for most students, haven't seen anyone struggle with this.
This pattern recognition technique also works well for teaching students how negative integer exponents work. They can see how decreasing the exponent by one is the same as dividing by the base, and continue that pattern into the negative integer exponents.
This also works for showing why factorial(0)=1
the first one is wrong
@@imjustagirl18 it was prob just a typo
The fact that Negative × Negative = Positive doesn't not make sense, obviously.
why
@@Fire_Axus Re-read his statement carefully xD
amazing, +2
This doesn't make a unreasonable reply, -(-1) @@quixotix9540
i don't think this comment isn't the best
I like the 3rd proof, because it is intuitive and sets up a foundation for thinking of numbers in terms of directions, which is incredibly useful when learning immaginary numbers.
If a negative means a 180° rotation, and rotations add up, then it is intuitive that i=sqrt(-1) represents a 90° rotation.
Very nice observation... an operator operating on a number set, some combine values while other encode rotation.. the way I saw it was the general idea behind all of the many concrete examples worked through in school. And life
That's funny, I actually thought the 3rd proof was by far the hardest to understand 😅
@@Erlewyn That proof was my reasoning that led me to realize that -1x-1=1... In Primary 3, by myself, without any input from my pretty bad teacher.
but there is nothing "directiony" about real numbers other than order. it feels like something act on another because an operation sends 2 elements to a single and it could be interpreted as a set of actions by currying.
Thinking of the complex plane as a vector makes things way easier. Negation and multiplying by I is rotation, adding values is translation, and multiplying by positive reals is scaling
as a math major, my favourite one was still the number line explanation. if you thing of multiplying by negative as a reflection, reflecting twice just gives the original.
As a non-math person, it made sense to me too ... but only once I started to think about imaginary numbers. Because a follow up question would be "Well, what if I do a half rotation?"
@@PhotonBeastthen things get complex
As a high-school math teacher, that's what I used in class.
@@PhotonBeast its complex but i can do it
I like how straight to the point you are
The proof for non conmutative rings is the same, except we'd factor that -b at the right 😁
Thanks, I accidentally did it like that the first time and got very confused about where I allegedly used the commutative property😂😅
i was 100% expecting for you to pull out the complex plane.
Which is what I'd use to explain it. If you view multiplication by a negative number as a 180° rotation on the pos/neg number line, suddenly it doesn't seem so weird that i exists, that i² is -1 or that multiplying by i is an orthogonal movement.
Imaginary numbers being dubbed “imaginary” will never not be one of the greatest unsung tragedies of modern mathematics, science, and philosophy! There should have been a greater push to re-dub them as “lateral” numbers and introduce the lateral number line and number/rotational plane far earlier in math instruction (or at all since many don’t even get to that point!)
@@jemm113 ok that makes sense
Level 6: category theory. Guaranteed to make any simple fact mindbreakingly difficult
The multiplication of two negative numbers being positive clicked with me during my complex analysis course, akin to the level 3 you presented. In particular, representing negative numbers with euler’s formula and showing that multiplication leads to an angle of 2pi, which corresponds to the positive side of the real number line.
Does this concept apply to complex numbers as well?
The enemy of your enemy is your friend
machiavelli would be in awe with your IQ
Still my enemy.
😅😅
Soon you will find out it cuts both ways
If you turn backward and walk backward, you are walking forward in the original direction
The reversed curse technique
Sir, I'm a 10th grade student from The Philippines and I've been watching your videos since last month. Looking forward for more interesting videos and thank you!
I'm glad you like them!
You definitely deserve a lot more views. Love these '5 levels' of math topics 👏
Here's an approach from the set theoretic construction of integers built on top of set theoretic construction of natural numbers, natural number addition and natural number multiplications (no subtractions yet):
You can define an equivalence relation ~ between a pairs of natural numbers: (a, b) ~ (c, d) if and only if a + d = b + c.
To prove that it is in fact an equivalent relation I will prove its 3 defining properties:
1. Reflexivity: a + b = a + b, therefore (a, b) ~ (a, b)
2. Symmetry: a + d = b + c if and only if b + c = a + d, therefore (a, b) ~ (c, d) if and only if (c, d) ~ (a, b)
3. Transitivity:
Assume a + d = b + c (a, b) ~ (c, d)
Assume c + e = d + f (c, d) ~ (e, f)
Therefore a + d + c + e = b + c + d + f
a + e + c + d = b + f + c + d
a + e = b + f (a, b) ~ (e, f) (This last line is based on x = z x + y = z + y which is trivial)
Then, you can define equivalence classes under the ~ relation. Let's use the notation [(a, b)] to represent the equivalence class with (a, b) in it.
So, for example, the equiv. class that contains the pair (1, 3) will also contain (2, 4) because 1 + 4 = 3 + 2, will also contain (0, 2) because 1 + 2 = 3 + 0, etc.
The underlying motivation here is that if you swap a + d = b + c around, you get a - b = c - d, and [(0,2)] represents 0 - 2 = 3 - 4 = -2, but without having to have subtraction and negative numbers already defined.
Then you can define positive integers as any equivalence class that contains (x, 0) where x is a non zero natural number.
You can also negative integers as any equivalence class that contains (0, x) where x is a non zero natural number.
The proof for how the set of positive and negative integers don't intersect as a little sanity check for these definition, by contradiction:
Assume A is a positive integer that is also a negative integer.
Therefore A can be represented as (x, 0) where x is a non zero natural number.
Therefore A can be represented as (0, y) where y is a non zero natural number.
That is, A = [(x, 0)] = [(0, y)]. That is, (x, 0) and (0, y) is in the same equivalent class A.
That is, (x, 0) ~ (0, y), which happens if and only if x + y = 0 + 0 = 0, which is a contradiction since both x and y are non zero natural numbers.
Therefore A cannot exist.
Since natural number addition and multiplication is defined, you can define integer addition as [(a, b)] + [(c, d)] = [(a + c, b + d)] and integer multiplication as [(a, b)] . [(c, d)] = [(ac + bd, ad + bc)].
As far as I know, this is the canon way to construct the set of integers along with integer addition and integer multiplication.
Then, from this definition, you can have two integers A = [(0, x)] and B = [(0, y)] where x, y are non zero natural numbers, therefore A and B is negative.
Multiplying integers A and B, you'd get the integer [(0*0 + xy, 0*y + x*0)] = [(xy, 0)]. This integer fits in the definition of positive integers that we've defined above.
The rational numbers are built from equivalence classes of pairs of integers, the same way integers are built from equivalent classes of natural numbers, but with a different equivalent relation. The proof for that from definition wouldn't be as exciting because it would mostly use the result proven above in one way or another, and for real numbers (which are btw constructed in an entirely different way as Dedekind cuts) it's just an identity in real number multiplication's definition, which is not really exciting to be honest.
Hope I haven't made mistakes lol.
Near the end of your second paragraph, you wrote a+e=c+f instead of a+e=b+f
@@globglogabgalabyeast6611 thx for the correction i've fixed it now
"I enjoyed your breakdown of the different types of rings, like commutative vs. non-commutative. Very informative!"
Wow, I feel like I've finally leveled up. This helped me actually understand how a proof for simple concepts works. After spending the last several months studying modular arithmetic and rings, it's finally clicking.
Nice intro to rings also.
If you imagine the number line as a sphere, the operation of multiplying by negative one is equivalent to rotating the sphere downwards (if using i, negative i is upwards) passing through the complex modulus a la e**iπ. Factoring out (-2)(-3) to -1(2)•-1(3), you end up at positive six, and travel through both poles in the complex plane to arrive at positive 6.
Great video, not at all surprised you blew up so quickly. Hopefully the begining of a long UA-cam career!
Thanks so much!
I feel it's easier to understand intuitively through division. Negative number divided by a negative number is positive because thats how many times it can be divided. And division can be easily shown as multiplication.
Not sure why you think that. The question from a class would be. 9 divided by negative three how can you get a negative answer in division?
@@johnmaguire2185 I think it in terms that negative fraction is a positive fraction multiplied by -1.
Even as an engineering student who has a ton of math experience I still think about it as a (-) x (-) = + since one negative sign rotates and combines with the other - to physically create a + sign. Than a (-) x (+) = - since the - erases a line from the plus leaving you with a -. Does it make any sense…. no not really. But did it make sense to 8 year old me… yup!
I like the expansion of understanding of mulitplication which also holds for complex numbers. A number can be expressed as a length and an angle. A positive number has angle zero, while a negative number has angle pi (or 180 degrees if you don't like radians). I.e. the number 1 can be expressed as length = 1, angle = 0. And -2 could be expressed as length = 2, angle = pi. Multiplication is then an operator which multiply the lengths (which are positive and purely real) and add together the angles. So (-2)(-3) yields length = 2*3 = 6 and angle = pi+pi = 2*pi (which is 0 since angles are mod 2"pi).
Interestingly, this holds for complex numbers as well. I.e. i is length = 1, angle = pi/2. With that, it's pretty simple to intuitively perform multiplication within the complex space.
I was actually thinking of this exact question earlier today when a student asked me what happens when you multiply negative numbers. I got up to level 3 on my own, but level 4 was my sweet spot. The concept of rings was interesting, but really only useful for math majors.
An attempt at a proof by contradiction (I have never taken a formal proofs class, so I won’t be surprised if I have made a mistake somewhere)-
Let a, b, and c be arbitrary, strictly positive numbers > 0. Assume that (-a)*(-b)=(-c) for some a, b, and c.
We can factor out a negative one from some of these numbers. Rewrite (-b) as (-1)*(b) and (-c) as (-1)*(c) :
(-a)*(-1)*(b) = (-1)*(c)
divide both sides by negative 1:
(-a)*(b) = (c)
(does this step implicitly assume that negative times negative is positive? not sure)
an this point, we can see that the left side must be negative, and the right side is positive, since a, b, and c were chosen as strictly positive. this is a contradiction, meaning that negative times negative cannot equal negative!
So I've studied a little bit on formal proof's but I'm not a mathematician (just studying Comp Sci) so could also be wrong here.
You can use this proof to show -1/-1 = 1 without assuming negative * negative is positive:
let a be a non-zero number:
a/a = (a)^1 * (a)^(-1) = (a)^(1 -- 1) =a^0 = 1
Thinking about your proof also made me think of a proof, although I may be doing something I can't do:
let x,y be any positive (>0) real numbers:
assume -x * -y 0)
By our above proof:
(-x)/(-x) = 1
(-x) * 1/(-x) = 1
we know 1/n != 0 for any non-zero real n, so 1/(-x) is not zero.
Since -x is negative, if 1/(-x) were positive then:
(-x) * (1/-x) < 0
Thus:
by exhaustion 1/(-x) must be negative
Thus
1 0
Level 4 is level 5 with numbers.
I always thought it was a function with 2 inputs and with that you can define any list of relationships you want with a two input function. So it was because we say it is so.
I like making a times table for 1 to 9 and pointing out how the rows and columns increase regularly.
then start extending the table to the other quadrants going from (x,y) to (x-1,y) and (x,y-1) a step at a time
Using the properties of imaginary multiplication in the 2d plane, where multiplying two numbers looks like adding their angles (from the x-axis) and multiplying their distances from the origin, multiplying two negative numbers looks like multiplying their distances, then adding 180°+180°, for 360°, or 0°
I thought he was going to talk about the complex plane, multypling two numbers with angle π (negative) give us a number with angle 2π (positive) because we add the angles.
That's one way I immediately thought about it
I find that it is easier to understand with the complex numbers. If you multiply a complex number by i, it is shifted by π/2(90°):
1 * i = i
i * i = -1
-1 * i = -i
-i * i = 1
And if you now multiply by -1 instead of multiplying by i, the angle of rotation doubles, i.e. π/2 becomes π(180°).
It's good to get pupils to derive it themselves. Create a a coordinate system and at each point in the first quadrant get them to enter a number which is equal to the product of the x and y coordinate. They will then see arithmetic sequences (times tables) when considering constant x and constant y. Ask them to extrapolate sensibly into the 2nd and 4th quadrants, and then again into the 3rd quadrant. To keep the structure of arithmetic sequences they will see that all the entries in the 3rd quadrant are positive.
I used to think of the multiplication by -1 as rotating the location vector to the number I’m multiplying on the number line with -1 by 180 degrees around the 0 point. Thus 1 times -1 times -1 yields a rotation by 360 degrees, putting me back to where I started, positive 1. This visualization also comes in handy when trying to get a grasp on the complex plain and understanding why the square root of -1 is orthogonal to the number line. Because instead of rotating by 180 degrees, you just rotate by 90 degrees. And Sqrt -1 times Sqrt -1 is rotating by 90 degrees two times giving you the 180 degrees of the multiplication by -1. There you go.
Here is one using the construction of the integers:
Integers are defined as equivalence classes on N^2 where (a,b)~(c,d) if a+d=c+b.
All equivalence classes can be written in the form [(a,0)] or [(0,a)]. Ones in the form [(a,0)] are just denoted as a and ones in the form [(0,a)] are denoted as -a
Multiplication is defined as [(a,b)] * [(c,d)] = [(ac+bd,ad+bc)].
We have -1*-1=[(0,1)]*[(0,1)]=[(0*0+1*1,0*1+1*0)]=[(1,0)]=1
Turn around
Turn around again
Woah I'm facing the same direction
What about positive numbers?
Don't turn around
Don't turn around again
Woah I'm facing the same direction
i feel kinda stupid for having to look up what distributive propertys are but I feel really good for understanding it now.. I cant believe I went so long trying to rearange formulas with this pretty basic concept but Im glad I got it now
The way I see it is this:
On a number line, each sides “negative” direction is towards 0 and through the other end of 0. 2-3 being 2 towards 0 and 1 through 0 from the right side. And each sides positive direction is farther away from 0.
Thus,
2 x 3 = 2 + 2 + 2
Adding 2 in the positive direction (right) 3 times, getting further away from 0.
-2 x 3 = -2 + -2 + -2
Adding 2 in the negative direction (left) 3 times. Think of it as starting on the right side of 0 and going through 0 in the negative direction (towards 0).
-2 x -3 = -2 - -2 - -2
Subtracting 2 in the “negative”direction 3 times. Think of it as starting from the left (negative) side of 0, and going towards and through 0, in the left sides “negative” direction, which is really the right sides positive direction. Thus meaning that it really is just going positively
Another video from the goat! Just solved one of IMO questions and here is my reward video!
Thanks, and nice work with the IMO question!
A math teacher explains all the properties of multiplication to the class and concludes: "While two negatives make a positive, there is no way two positives make a negative"
And from the back of the class, someone comments: "Yeah, right".
Very interesting video! Your explanation of the ring is incredibly clear, and most importantly, it makes clear a very important thing in all of mathematics: apparently, all mathematical operations are determined by their properties. The same can be said about physical quantities - they are all nothing more than numbers, but their key difference from random numbers is that they have properties that reflect processes in the physical world. Definitely like!
Thanks so much, I'm glad you liked it!
The first thing i thought of was rotations in the complex plane. Two 180 rotations just end up at the same place
Same
The rules for rings are defined that way because we want them to be useful and generalize the idea of integers, i.e. counting stuff like sheep and yes, debts too.
Is it just me, or is a ring just a vector space? I swear that’s what I learned is the definition of a vector space
I like to think of this from a statistical standpoint. You can combine + and - in 4 different ways. (+)x(+), (+)x(-), (-)x(+) and (-)x(-). If you threw a dart randomly at the number line then your answer would have a 50/50 to be (+) or (-). Each one of these 4 combinations accounts for %25 of the answers. Starting with the axiom that a +x+=+ which is %25 of your answers. Then that +x- must also be %25 of your answers and that -x+ is the same so together count for %50 of the answers therefore must be -. Leaving the last %25 (-)x(-) which must be +.
It's not a proof, but intuitively works:
If you can agree that multiplying bij -1 is just flipping a number's sign. Which I think makes sense.
If you also agree that (-a)(b) =-ab. Which also makes sense, because this is just adding -a b times: (-2)(3)= (-2)+(-2)+(-2). I.e. shifting three time 2 to the left on the number line.
Then it follows: (-a)(-b) = (-1)(a)(-b) = (-1)(-ab) = ab
My favorite is multiplication as rotation of a vector in the complex plane. Multiplying by a negative is multiplying by 180°. To multiply a negative by a negative is to rotate by 360°, which is identical to 0°, or positive.
I used to think of it as:
Positive (positive) ->> the 2nd positive tells the first to keep its value so its ->> positive
Positive (negative) ->> the 2nd negative tells the first to reverse its value ->> negative
Negative (negative) ->> the 2nd negative tells the first to reverse its value, making it positive
Or
2 same = positive
2 different = negative
Pos (pos) = pos
Neg (neg) = pos
Pos (neg) = neg
Neg (pos) = neg
when I was in elementary school, I thought of it as flipping the whole number line, so flipping it twice is the same as the original
if you apply the inverse of negative to the number line, it's the same as flipping in the other direction ends up looking the same as applying the negative to it, which explains why 1÷(-1)=-1
after learning about complex numbers, I thought of it is rotating the number line instead of flipping
You can build so many proof of this law, using different level in math's.
Saying that's exponential is a group morphism from C to C* with 2piZ as a kernel.
Or using the Euclidean division in the polynomial ring, writing X^2=(X-1)(X+1)+1 and that - 1 is the unique root of X+1 in Z[X].
Here are the proof that comes immediately at my mind.
I was taught multiplication of positive and negative numbers with this idea:
If friends are positive and enemies are negative:
-The friend of your friend is your friend
-The enemy of your friend is your enemy
-The friend of your enemy is your enemy
-The enemy of your enemy is your friend
Let's represent any positive value N as a vector along the +x-axis. It's coordinate pair vector representation will be (N, 0) for all values where it's tail begins at (0,0) and its head ends at (N,0). Now we can rotate this vector N by 180 degrees. For example if we rotate the value 5 denoted as (5,0) and rotated it by 180 degrees by the applied transformation, it will now be located at (-N, 0).
There are several types of transformations:
Degree of Transformation: Type of Transformation: Associated Operator: Geographical Representation:
0D N/A - No-Op Identity +(0), *(1), ^(1) Point, Locale.
1D Linear translation or displacement Addition: + Line, Distance or Magnitude
2D Scaling, Shearing, and Rotation Multiplication: * Distance, Angle, Bounded Area
3D Multiple Scaling, Shearing, & Rotation Exponentiation: ^ Distance, Angle, Bounded Surface Area, Volume
...
When we rotate a value by 180 degrees, this is similar to multiplying it by -1. If we start at the vector (-3,0) and rotate it by 180 degrees same as multiplying it by -1, we end up at (3,0). We can then take this rotational transformation aspect of the multiplication operator and extend it to any arbitrary value as:
(+A)*(+B) = +(AB)
(+A)*(-B) = -(AB)
(-A)*(+B) = -(AB)
(-A)*(-B) = +(AB)
With this we can let B be equal to A and thus we can represent it as A^2. Why am I using this type of representation? Since multiplication is defined as repeated addition, it is still a transformation. If we take 3*2 along the +x-axis. Our initial vector (3,0) will then be displaced to (6,0). This is still a linear transformation however, since this is repeated addition, this is also the scaling version of multiplication. 2D Operations inherit 1D Operations. We can also form angles by separating this into 2 vectors. The first vector A is along the +x-axis which is still (3,0). The second vector starts at (3,0) and shifts up to (3,2). Then we have to translate from (3,2) to (6,2). From here we can complete the parallelogram. The area of this parallelogram will be 6 units squared. And the far right vertice when projected straight down to the +x-axis will also land on (6,0). We can use the pythagorean theorem to find the length of the 3rd leg which becomes sqrt(13). And we can use the arctan(2/3) which gives an angle at the origin between the initial vector and the hypotenuse of about 33.69 degrees. This a combination of algebraic, geometric, trigonometric, and vector representation of multiplication and how this operator implies scaling, rotation, and area.
Now if we look at the properties of squares, y = x^2 and we try to solve for y. We end up our quadratic equations. When we try to find the roots, only real roots exist +/-(sqrt(n)). However, when we try to think of what two numbers multiply to give sqrt(-1) we end up with i which is typically called imaginary. Yet, it's far from that. What is i? It is the fact that i^2 = -1. Within the reals, when both signs are the same the result is positive. When either sign is opposite the result is negative. This is because multiplication of -1 is a rotation of 180 degrees or PI radians. Multiplication of 1 is a rotation of either 0, or 360 degrees, or 2PI radians. The values of 1 and -1 are parallel but are opposing. So where does i come into this? Well, i is the rotation of a value by 90 degrees or PI/2 radians. Let's look at this way: i^2 = -1 and i^2 = -1 and -1 * -1 = +1. So if i^2 = -1, then i^4 = +1. We just went around the unit circle within the complex plane.
Multiplication is repeated addition by definition sure, but it's also rotation. This is one of the reasons why there's a direct relationship between the cosine function and the dot product. This is why we have reflections, and symmetry. We have symmetry because of the cosine function which is also an even function. If we horizontally translate the cosine function by 90 degrees or PI/2 radians, we end up having the sine function. They will map onto each other because they are orthogonal to each other. And the sine function is why we have asymmetry because it is an odd function.
This becomes more evident when one starts to work with Fourier Series, and Fourier Transforms and begins to fully understand Euler's formula.
I use the last method with all students from 11 years old up. I don't talk about ring theory just algebra after they have learnt to expand brackets and to factorise.
You've described the ring theory and the twelve hours ring, but you've used the same proof from step three again in step five, ie you didn't need to use the ring theory at all, but thanks for explaining the interesting theory though 😊😊
a*e^(i*pi)*b*e^(i*pi)=a*b*e^(2*pi*i), assuming a and b are positive real numbers, as a magnitude cannot be negative, and multiplying by zero will yield in a non-positive and non-negative solution.
In the intuitive example, I think the person is left with 6 cookies, because it is equivalent to say 6 cookies are put into my 🍪 jar or left in my possession outside of a jar. I think of multiplication and division as the same as adding full containers of the same size. For example, 12 eggs is the same as a carton of a dozen eggs. Multiplication is just adding up cartons: 1/2 of a carton is splitting the carton into two, each with 6 eggs, but I still possess all 12 eggs. The trick is that we are implying that changing the size of a group also means we lose possession of some of the contents and groups. But perhaps we could interpret x/0 to mean the information possessed is ungrouped...and so has to be counted individually.
Haven’t seen level 4/5 done like that before. I would go more like
1. -(-a) = a. Any definition I can think of is obviously symmetric, e.g. if b is called -a when a + b = 0 then a is also -b.
2. (-a)(-b) = ? = -(-a)(b) = ab if you can justify moving the -…
3. -a = (-1)a. No trick: Just check that (-1)a is -a. (-1)a + a = (-1 + 1)a = 0a = 0.
4. 0a = 0. Same way in video :-)
Got randomly recommended this and ended up watching a bunch of your videos. Hope ya keep making them! I majored in math but I think your videos are easy enough to understand and just about anyone can appreciate them. Easiest subscribe in a long time
Thanks! I really appreciate that
@@DrSeanGroathouse Surprised to see such a young channel, first video is only 2 weeks ago like DAMN
Yeah, the way I think about negative operations is basically via boolean algebra, as it’s easier to reason about values encoded on a single bit. You can draw a 2×4 table with all possible bit combination, and realize that this negative operation is basically the same as the NOT boolean operator. A true value is +P, and a false value is -P.. Negating simply flips the sign. So -(+P) is -P, -(-P) is +P.
Then, -a * -b is the same as first doing a * -b, and flipping the result sign, which ends up positive.
I saw an intuitive way for this: Assume we already figured out positive times negative = negative, and a + (-a) = 0 (not gonna put here how because would be too long). Now, imagine -2(6+(-6)), this is -2(0), which is 0. If we distribute the -2, we get -2(6) + (-2)(-6), and this is equal to 0. Let's call (-2)(-6) x to make this shorter. We know that-2(6) = -12, so we get -12 + x = 0. This means that x has to be 12, meaning (-2)(-6) = +12
Btw, I used to think about this like you said in the number line: adding negative is going in the opposite direction, multiplying negative and positive is also just going in the opposite direction, and multiplying 2 negatives is just going to the opposite opposite direction, which is the original direction
Using complex numbers.
Do a 180 (× -1)
Do another.
You are facing back to when you were.
----
Ring Theory proof works but the one above is my favorite as its deeper then it looks but easy to explain.
Edit: Guess it's covered.
(0-1)(0-1) = 0² -2×0×1 + 1² = 1 use this definition to do the rest
In complex number plane -1 = e^(pi*i) is rotating 180 degrees. So (-1)^2 = e^(2*pi*i) = e^0 = 1
Removing sadness leaves you happier
I was literally in the shower thinking „what is the actual mathematical proof of neg x neg = pos” and after i came out of the shower and open yt this showed up.
I think you can demonstrate it by contradiction. A non zero product of two negative numbers is either positive or negative . If the product of two negative number was negative, then by multiplying it by a negative number, you would get another negative number, and by multiplying by a positive number, it would be negative too.
So if it was negative you would get:
1*(-a)*(-b)=(-1)*(-a)*(-b)
which leads to either -1 = 1 or (-a)*(-b)=0. Both of them are false by hypothesis (on real numbers or Q or Z). so the product of two negative numbers must be positive.
notice: ab+a(-b)+(-a)(-b)= ab+a(-b)+(-a)(-b)
Factor -b on left side , factor a on right side
ab+(-b)(a-a) =a(b-b) + (-a)(-b)
i.e. ab+(-b)(0) =a(0) + (-a)(-b)
hence, ab = (-a)(-b)
Level 6: The positive reals P are a subgroup of the multiplicative group of reals. The reals are then partitioned into two cosets, P and
-P (negative reals). The quotient group is then the cyclic group of order 2, and negative * negative = positive. By definition of multiplication in the quotient group, this means that a negative real times another negative gives a positive.
I believe this isn't circular because the only things you have to know about the group is that the negative reals and the positive reals are disjoint and their union gives R, and that the positive reals are stable.
The first meaning that we give to multiplication is adding a number repeatedly. The two numbers involved in the multiplications have very different meaning, the first one indicating which number will be added repeatedly and the second one indicating how many times is this going to happen. From the significance point of view factors are not interchangeable. Negative numbers are introduced to manage situations that are hard to manage using only natural numbers, in fact you would need to use two sets of natural numbers and quantities of one of these sets cancel out with corresponding quantities from the other one. So i think that before answering what -ax-b means we should answer what meaning can be given to repeating addition a negative number of times. But it turns out that repetitions are also quantities that can be sometimes considered of two different kinds that cancel each other: in your second example you mentioned a friend who repeatedly gives or repeatedly takes away the same quantity, there can also be repetitions from this point in time to the future vs repetitions in the past to this point in time: form example if my account now is 0 and a friend has deposited 5 dollars in my account for the past 3 days, it means that my account was -15 dollars 3 days ago. Using the same example if my account was debited 5 dollar for the las three days and now is 0, it means that 3 days ago y had 15 dollars, thus -5x-3=15. My point here is that you basic example is more basic than the first because the first involves continuity and the second one doesn't, but also because it better addresses the questions young students have when they try to reconcile this new "rule" with what they previously knew about this operation.
At my university i was taught since you can prove -x=(-1)x=x(-1), the fact that multiplication in a ring is associative means that you can "migrate" the minus signs in the expression (-x)(-y) to the front, and get (-x)(-y)=(-1)(-1)xy. Which can then be shown to be equal to xy.
Yep. You can also prove -x=-1x=x(-1) from (-a)(-b) = ab as well.
The way how I think of negative numbers is this:
I picture a graph of a number line in my head.
Positive numbers are at the right while negative numbers are at the left and zero is at the middle. When I add a number with positive value, the position of said value moves to the right. When I add a number by a negative value, the direction is reversed.
When I multiply a number by a positive number, the position of the number that is multiplied remains the same, then the value gets multipled and moved. When I multiply a number by a negative value, the position of the number being multiplied gets "mirrored", that being the number being converted into a negative value.
When the multiplier is greater than 1 or less than -1, the value moves away from 0. When the multiplier = (-1 , 1), excluding zero, the value is moved towards 0. When the multiplier is zero, the value moces straight to 0.
For division, the direction is reversed compared to multiplication. However, in the case where the divisor is 0, if the multiplied number is not 0, it cannot be any value. If the multiplied nunber is 0, it "technically" can be any number. However, that would be useless and a worthless excuse to use this number to answer every mathematical problems so it is written as "undefined" instead.
For exponentiation, specifically when the exponents are even numbers, "mirrored" values (Negative values) and "normal" values (Positive values) are presented. Roots are the reverse of exponentiations except that if the exponent is an even number, it can only represent one of the possible value (Example: √4 = 2 and only 2, while -√4 = -2 and only -2).
I might have explained the concept of negative values as the literal traditional definition of it but personally, I understand them this way. There might be some flaws in them though.
1:05 This is what made me understand! Thanks! but I'm gonna watch the rest.
So Level 5 is just a generalization of Level 4, or Level 4 is an example in the ring of integers of Level 5?
I would use -1 = e^(pi*i). So (-1) * (-1) = (e^(pi*i)) * (e^(pi*i)) = e^(2*pi*i). And by looking at the unit circle we can see that e^(2*pi*i) = 1.
Although I guess complex numbers already rely on (-1) *( -1) = 1 so this might be circular.
0 = 0×0 = (1 - 1)×(1 - 1) = 1×1 + 1×(-1) + (-1)×1 + (-1)×(-1) = 1 - 1 - 1 + (-1)² = -1 + (-1)²
Adding 1 to both sides gives (-1)² = 1.
Since a + (-1)×a = (1 + (-1))×a = 0×a = 0 = a×0 = a×(1 + (-1)) = a + a×(-1) subtracting a on both sides yields (-1)×a = ax(-1) for all a in R. Thus, -1 commutes with all a in R.
Now, (-a)×(-b) = (-1)×a×(-1)×b = (-1)×(-1)×a×b = (-1)²×a×b = 1×a×b = a×b by what was shown 1st.
So I had to show that -1 is in the Centre of (R,×) but after that it's just 1 line of easy Algebra.
I didn't use brackets since +, × are associative though to be precise I should've propably used them.
Really a nice explanation..i like math a lot, now teaching my middle schooler and elementary...your videoa are invaluable
1:40 if my friend takes 3 of my depths It's not the same as multiplying -3 by -5
It's funny because when you said Ring Theory I was like what the fuck is this I've never heard of that in my life.
And as soon as you started multiplying variables together I realized immediately what it was.
I was never give the term ring theory before so I was at a loss lmao.
*level 6: reverse psychology*
imagine you have a child thats rebellious and does the opposite of everything you say (negative)
if you tell that child to clean his room (negative * positive), he will not clean his room (negative)
if you tell that child to NOT clean his room (negative * negative), he will clean his room (positive)
Don't forget about the complex plane and Euler's Identity. Negative real numbers have an angle of pi, so their products have an angle of 2 pi, which brings you right back to the positive real numbers.
multiplying by i in the complex plane is a rotation of 90 deg. multiplying by i twice is the same as multiplying by -1. Do that again and we rotate a full 360 getting back to a positive number!
I always associated this with the phrase "agree to disagree"
Lets say agree is positive and disagree is negative
Agree to disagree is agreeing to disagreeing which makes it disagree resulting into a negative
Disagree to agree is disagreeing to agreeing which makes it disagree resulting into a negative
Agree to agree is agreeing to agreeing which makes it agree resulting into a positive
disagreeing to disagreeing is disagreeing to disagreeing which makes it agreeing resulting into positive
dunno if its more complicated tho
the way I always understood is this: 5x10 is five tens, 50. -5x10 is negative five tens, -50. -5x-10 is negative five negative tens. a negative negative is a positive
That moment he reached Level 5 Ring and you realized you have to take dog for a walk ...
this might confuse some people but the way i saw it is
-2 times 2 is -4, its -2 happening two times. -2 times -2 is just the number not happening, twice. -2 not happening -2 times is a positive 4
Thanks Dr. S, I wish you luck in your growing channel 🥳
when any complex number (real numbers are a subset of the complex numbers) is multiplied by another complex number their magnitudes are multiplied (this will always be a positive real number so no need to fight the negative multiplication basis here) and their phases are added (phase ranges from 0 to 2*pi with the modulus of any phase greater or equal to 2pi to bring it back) since the positive numbers have a phase of 0 0+0 = 0 and the negative numbers have a phase of pi pi+pi = 2pi since this is equal to or greater than 2pi then when we do the modulus it goes back to 0
its as simple as that we just gotta take a trip to the world of complex numbers
I keep it like this. So when somebody says something like two negatives eg: "I don't need nobody". So it says he doesn't need nobody meaning that he wants somebody. Aka: Negative times Negative = Positive.
I remember learning about multiplying by negatives and thought "if I multiply them together do you get positive" yea I was kinda a child prodigy
It's one of the simpliest problems to solve and understand
It is cool to see Level 5 after just learning about Vector Spaces in my linear algebra class. It was not taught to me as 'rings' the first 7 properties at 5:26 were the same as my textbook, however, the 8th one was '1v = v'. Anyways, cool to see and nice video.
keep up the good work...
May the mighty bless you..
Visualization, realization, numericization, abstraction, arbitration
Your friend was asked how to pay his debts but took "triple it" and pass it to the next person to a wholecbew level
I used complex numbers once.
A mathematics major wanted a geometric proof that really made it clear how and why multiplying by by a negative does what it does. Something like you might see on 3blue1brown.
So I said you start with 1 on the complex plane.
Now multiply 1 by i and you see how it rotated that 1 90 degrees to the i position.
Now multiply by i again and it rotates another 90 degrees to the -1 position.
This multiplying by i twice is that same multiplying by -1.
What you get is that multiplying by a negative is the same as rotating 180 degrees on the complex plane.
If you doubt this you can take smaller increments and watch as you march around the unit circle. It would probably be a fun animated video proof to watch.
what about sqrt(a*b) = sqrt(a) * sqrt(b) and why it doesn't hold for when a and b are both negative?
Well if the n is the square root of a real number m then n has 2 important properties:
1. n^2=m
2. Both the real part and imaginary part of n are nonnegative
For all a and b (sqrt(a)*sqrt(b))^2=sqrt(a)^2*sqrt(b)^2=a*b. So the first property is met no matter what
The 2nd propert is where things get complicated.
If a and b are both nonnegative then sqrt(a) and sqrt(b) are both nonnegative so sqrt(a)sqrt(b) is nonnegative. So 2nd property holds when a and b are both nonnegative.
If a is nonnegative and b is negative then sqrt(a) is positive and sqrt(-b) is positive so sqrt(a)*sqrt(b)=sqrt(a)*isqrt(-b)=i(sqrt(a)*sqrt(b)) has a nonnegative imaginary part (and a real part of 0) so the 2nd property still holds when a is nonnegative and b is negative.
By symmetry, when a is negative and b isn't the 2nd property also holds.
Now when a and b is negative you get something interesting.
sqrt(-a) and sqrt(-b) are both positive so you would have sqrt(a)*sqrt(b)=i sqrt(-a)*i sqrt(-b)=i^2*sqrt(-a)*sqrt(-b)=-sqrt(-a)*sqrt(-b). This is a negative so the 2nd property does not hold.
Overall this means that sqrt(a)*sqrt(b)=sqrt(a*b) if and only if a and b are not both negative.
Another method is to imagine that it's a slope. Were if slope goes back in x-axis and goes down in y-axis, it turns positive. Maybe didn't say it right but you do get the idea.
I don't get the idea bro
@@nicolasbenincampi watch a video about y=mx+b, should help to understand it more.
Awesome. I'm relearning math to get better for engineering. This helps so much, although simple it helps me actually understand the math vs just learning by memorizing formulas. Can you make one on why we set use zero property rule for quadratics? I can't understand the purpose of making the equation equal to zero.
My teacher just said there are two sides: allies and ennemis. ++ Is the allies of the allies = the allies, -+ is the ennemies of my allies = the ennemis etc. I thinks it's very easy to remember