Commutative property fails?

Algebra Level 1

I will attempt to prove that 0 0 = 1 \frac00 = 1 . In which of these steps did I first make a mistake by using flawed logic?

Step 1: We can rewrite 15 as 7 + 8 7 + 8 or 8 + 7 8 + 7 .
Step 2: This means that 7 + 8 = 8 + 7 7 + 8 = 8+ 7 .
Step 3: If we move one term from each side of the equation to the other side, we will get 7 7 = 8 8. 7-7 = 8-8.
Step 4: Dividing both sides by 8 8 8-8 gives 7 7 8 8 = 1 \frac{7-7}{8-8} = 1 .
Step 5: Since 7 7 = 0 7-7= 0 and 8 8 = 0 8-8 = 0 , 0 0 = 1 \frac00 = 1 .

Step 1 Step 2 Step 3 Step 4 Step 5

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33 solutions

step 4, it is illegal to divide by 8-8 because 8-8 equal zero

Moderator note:

With division by zero, the argument is often made along the lines of:

You can't divide 10 by 0, because that's equivalent to asking "0 times what is 10?" which clearly doesn't have an answer.

This isn't entirely satisfactory; imagine the same rules were applied to the square roots of negatives:

You can't take the square root of -4, because that's equivalent to asking "what times itself is -4?" which clearly doesn't have an answer.

Yet we define a new number, i , i, that allows this to be resolved. Why can't we define a new number that simply is the result of dividing by zero?

The answer is with "proofs" like the one given in this problem. By allowing division by zero (or a symbol for it) it is possible to prove any number equals any other number. (The shortest such proof starts with a × 0 = b × 0 a \times 0 = b \times 0 which is true for any numbers a a and b , b, then divides both sides by zero, leaving a = b . ) a=b.) Once all numbers equal each other, the mathematical system breaks down entirely. This is the reason for the prohibition.

Thank you. I like that explanation because it shows me WHY maths (mathematicians??) adopts this rule. Then that makes you think about interesting things like alternative mathematical systems. Like, mathematicians say 'the universe IS numbers', but... which numbers? How do you know that your conventions of maths (which is what this is, yes? Conventions about how you will manipulate numbers)... how do you KNOW that it really matches any rules within the universe? AH. I guess like the rest of science. "Test it and see".

A V-B - 4 years, 1 month ago

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Well, to be nitpicky, the universe is actually everything but numbers. Numbers don't exist, just as operations, vectors and all the other brilliant mathemathical constructs invented to model the universe and it's behaviour, don't exist. Math exists purely for our understanding of all the nonsense around us we call "environment" or "space-time" or "whatever". Without maths, and all the other man-made non-tangible scientific constructs, our civilization would crumble, but life would surely go on, the moon would still be on the same trajectory, and black holes would still be... black, I guess. So yeah, our desire for understanding and knowledge actually adds to the system we're trying to understand, since we're inventing concepts that model it, and these ideas become part of the model. How cool is that :) To be relevant to the question: it is flawed logic, because we deem it false, and we say "all numbers can't be equal". But what if they are? In our model they aren't, and it works for us, but if quantum-sciences proved anything in the last couple of years, it's that you can't model everything, you can't predict everything, and you certainly can't apply such axioms, that seem universal, well, universally.

József Inczefi - 4 years, 1 month ago

I tell my students that when we ask the question, "What is 6/3?" We are asking, "If I have 6 cakes and divide then between 3 people, how many cakes do those 3 people get each?" 6/0 becomes the question, "If I have 6 cakes and divide them equally between 0 people, how many cakes do those 0 people have each?". It then becomes clear that the problem lies not with there being no sensible answer, but rather that the question being asked makes no sense! Of course, the only problem with this is that similar questions involving division by negative numbers and by fractions also make no sense in this context (but luckily my secondary school students have never picked me up on that yet!).

Steve Edwards - 4 years, 1 month ago

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Sir can you explain why division by negative numbers or reals is possible (note that you're constrained to the example of distribution of n n things among m m people)? XD

Tapas Mazumdar - 4 years ago

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It's a lot simpler, than you think. If 6/3 would mean having 6 items and diving it between 3 people, -6/3 would be owing 6 items and dividing the debt between 3 people. Then you just have to realize that 6/-3 is the same as -6/3 and -6/-3 is the same as 6/3, since dividing between -3 people just doesn't make sense. As stated, it's an explanation for second graders, don't try to wiggle more out of it than necessary and possible.

József Inczefi - 4 years ago

Step 3: If we move one term from each side of the equation 7+8=8+7 to the other side, we will have to move the same number and we would end up with -7+8=8-7=1 or 7-8=-8+7=-1 , and that would make step no3 the wrong move. Am I mising something here? Can someone please forgive my ignorance and correct me. Thank you.

N N - 2 years, 9 months ago

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7+8 = 8 + 7

-7 from both sides

8 = 8

or

7+8 = 8+7

-8 from both sides

7 = 7

This was my answer too.

Buuut

They're doing

7 + 8 = 8 + 7

-7 from both sides

(7 - 7) + 8 = 8 + (7 - 7)

-8 from both sides

(7 - 7) + (8 - 8) = (8 - 8) + (7 - 7)

Now everythings nulled out and we can remove a parenthesis from each side at will and as selectively as we want. Want to remove 1 (7 - 7) term? Go for it.

(8 - 8) = (8 - 8) + (7 - 7)

Perfectly valid. Let's remove 1 (8 - 8) term now.

(8 - 8) = (7 - 7)

Still valid.

This is the complicated set of mathematics that they performed in Step 3 to get (7 - 7) = (8 - 8)

Taylor Nonyabusiness - 2 years, 7 months ago

root of -4 is defined tho. It's the imaginary number of 2i by the definition that the number i is equal to the root of -1

Johan Augustsson - 2 years, 5 months ago

Step 4 is circular reasoning yes?

Mark Howe - 1 year, 7 months ago

so then what dose 0 divided by 0 =... shouldn't it be 0 or -0? ._.

Alexander Oricoli - 1 year, 6 months ago

You tried to proof that 0/0 = 1 by creating a situation where there is 0/0 (7-7/8-8) and then you just decided that 0/0=1.

Purple Crystal - 1 year ago

I don't know how I got here .

EVAN MCBREEN - 9 months ago

0/0 = 1 Just because it messes up your theory it does not mean it is wrong or can not be used in reality. If you use zero as the point of origin this makes a whole lot of sense. When you think philosophically at least. Only the point of origin would be able to create something out of nothing. This also means that any number multiplied by the origin returns to the origin (reclaim or return). But when you try to divide any number that is not zero by zero this number will remain itself (the self). If I was to put this into words I would say: "To find god you need to multiply for only god can create through division."

Alexander Verbruggen - 7 months, 1 week ago

4 = ± 2 i \sqrt { -4 } = \pm 2i .

. . - 2 months, 3 weeks ago

This proof could begin with Step 2, which is true because of the commutative property of addition --- so Step 1 is unnecessary.

Stephen Hanes - 4 years ago

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The question wasn't "what is unnecessary", it was "first ... mistake". Neither step 1 nor step 2 is incorrect.

József Inczefi - 4 years ago

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Yes, you are correct about what the question was. I was simply pointing out ANOTHER weakness in the proof --- so what is your problem with that? Simply put, the proof is also inelegant because it contains an extra, unnecessary step.

Stephen Hanes - 4 years ago

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@Stephen Hanes Most false proof flourishes such unnecessary steps, mainly in order to confuse less math-savvy readers. But yeah, now I get what you're pointing out, that the 1st point, and most importantly the number 15, has nothing to do with the seemingly logical flow of the proof. It's mostly a distraction. Sure got us distracted here :)

József Inczefi - 4 years ago

Moreover, there are no variables in this equation. The original equation had no solutions since there was nothing to solve for.

Vikas Gupta - 4 years ago

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you don't have to have variables for a proof problem -_- like 1+1/2+1/4+...=2 - no variables there

József Inczefi - 4 years ago

I agree with the comments by others that step 4 is the one that is wrong. We don't need step 5 to validate that we have already made an error in step 4. (7-7)/(8-8) is NOT equal to 1.

John Doe - 4 years ago

Wouldn't 0/0 simply equal all real numbers?

Nathan Zordilla - 3 years, 7 months ago

How do I post a solution? I am new to brilliant.org

Ralphy105 Lieblein - 3 years, 7 months ago
Lixin Zheng
Apr 30, 2017

In step 4, dividing by 8-8 means dividing by zero. This is not allowed, so the answer is step 4 .

Yep, if this was allowed, then we could just have easily concluded that 0 0 = 2 \frac{0}{0} = 2

Agnishom Chattopadhyay - 4 years, 1 month ago

so division by zero is undefined or infinity

Bithi Peris - 1 year ago

To move the term you have to do the function to both sides. So x+y-y=x+y-y once resolved makes x=x. So you lose the one function in this case. So before you make it to step 4 the math is wrong.

Lee Izatt - 4 years, 1 month ago

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Ummm, no. First off, as long as you don't make any illegal operations, like, let's say dividing with 0, or taking the square root of a negative number when working with real numbers etc., you are allowed to choose between expanding and simplifying the sides of the equation at your own desire. If you wouldn't be allowed to do as much, you couldn't actually solve a lot of equations. By continuing this train of thought, your analogy is not quite correct. In this case it was x+y=y+x which became not x+y-y=y+x-y, but x+y-x-y=y+x-x-y, and then it was resolved by leaving it with x-x=y-y. So up to step 4 everything is fine and dandy, and then you divide by 0, or in this case 8-8. THAT is the important takeaway here, that no matter how it's written, if it's value is 0, you can't divide by it.

József Inczefi - 4 years, 1 month ago
Charles Andrews
May 2, 2017

0 cannot be a denominator therefore cannot decide by 8-8

True, iPad error. Typing too fast and not proofing

Charles Andrews - 4 years, 1 month ago

divide, not decide :)

József Inczefi - 4 years, 1 month ago

Wouldn't 0/0 simply equal all real numbers?

Nathan Zordilla - 3 years, 7 months ago

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Yes and no.

As in, your statement is correct theoretically.

But x/x=1 ‘for all values of x’(EXCEPT ZERO!!!) And 0/x=0. Same deal here. And x/0 approaches plus it minus infinity.

It makes it inconsistent to define, and confusing.

Yes, you are correct. But because of it being all real numbers, it is inconsistent. So no. It’s undefined.

Zoe Codrington - 2 years, 6 months ago

That's an interesting idea. I think that any number except 0 does: x/0=∞. It's reasonable, because when the divider is smaller, the answer is bigger. For example: 5/10=0,5 , 5/5=1 , 5/1=5 , 5/0.2=25 and etc. But ∞*0=all real numbers. That way:

5/0=∞ 6/0=∞ ∞=∞ 5/0=6/0 5/0×0=6/0×0 ∞×0=∞×0 all real numbers = all real numbers

Just like this: x/2=5 x/2×2=5×2 x=10

That way you can divide by zero and still there's no situation where 1=2.

Purple Crystal - 1 year ago

I cannot divide if there is a typing error in there or not.

Shimon L - 3 years, 3 months ago

But that was what the equation was trying to prove as wrong so that isn't a valid response/solution. The working out is perfectly reasonable until Step 4 because 8-8 divided by 8-8 is 1-1 (I think), which would still need to be included as the equation doesn't make sense without it. This equation is making the incorrect assumption that this would work in the same way algebra does. It really doesn't. Instead, say something like "Although the equation is trying to show that 0 can be a denominator, the way in which it tries to go about it makes the contradiction that 0 cannot be the denominator." Sorry for the mini-rant, just wanted to explain.

Rhys Harding - 2 years, 2 months ago

Couldn't 0/0 be defined as the set of all real numbers then? Or if it includes imaginary numbers too, then the set of all complex numbers.

Pablo Backer Peral - 3 years, 7 months ago

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the value of a single simple operation, a division in this case, can't be "defined" as the set of some numbers, let alone all numbers. it would be literally saying, that something divided by something is everything. that's not how maths work. in a different mathematical system it could be accepted, but then most of the axioms that we usually work with, and even the meaning of operations on numbers or numbers themselves, would change or lose meaning. ;tldr you can't just invent new math concepts and expect them to work with the old ones :)

József Inczefi - 3 years, 7 months ago
Lucas Jalaß
May 4, 2018

You can neither divide by 0 or multiply by 0 in order to simplify, here is why:

(just for understanding) 2 = 1 (i know this is false but will get there later) because we are technically allowed to multiply with 0 -> 0 x 2 = 0 and 0 x 1 = 0

so ... technically its possible but if we wanna simplify we get this:

2 = 1 | x 0

0 = 0 q.e.d. (Quo errat demonstrator) obviously doesnt work ... so we are not allowed to devide and multiply by 0 to simplify ... that does not mean we are not allowed to divide or multiply by 0 at all. I say 0 0 \frac{0}{0} = {-∞;...;∞} So here we go!

0 x 9 = 0

0 x 32423,23423 = 0

0 x 0,00001 = 0

so 0/9 = 0 ... and now comes the problem 0/0 = 9 is not correct ... it is just partly correct, because it is one solution out of many!

0/0,00001 = 1 ... same thing 0/0 = 0,00001 is not correct ... it is just a part of the solution.

now this messes with a few rules... but not entirely

for example: every number divide by itself is 1 -> a a \frac{a}{a} = 1 but now we have to make an exception a ∈ Q and a ≠ 0 (because we cannot divide by 0 for this rule)

ok now partly it is actually correct 0/0 can be 1 because 0 x 1 = 0 but it cannot ONLY be 1 thats why we cannot say 0/0 = 1

making it simple now: every number (a) multiplied with 0 equals 0 -> logically 0/0 equals every number (a)

example: I have 0 pieces of cake and I invite (a) number of people, then everyone will get 0 pieces (0 x a = 0) it doesn't matter how many people will come

now i divide by 0: (0/0 = a): I have 0 pieces of cake and the rule that everyone that got invited and came will get 0 pieces of cake -> I don't know how many people will come but it actually doesn't matter how many people will come.

and now it is clear why you cannot divide by 0 in order to simplify -> it just adds an infinite ammount of solutions to both sides (in case you divide 0 by 0)

and multiply with 0 would make it 0 solutions


now this is maybe hard to understand and I think the reason is the "=" equal symbol so let me try to explain:

4/2 = 2/1 mathematically this is correct but it doesn not really look equal does it?

2 = 2 simplified it looks better right?

√4 = 2 this is also just a part -> just like 0/0 = 1

√4 = {2; -2} and now look at this! -> 0/0 = {-∞;...;∞}

we have to define what we want -> √4 = b and (b ∈ N) then b = 2 but if (b ∈ Z) then b = {2; -2}

now for my cake example: '0' cakes 'x' (multiplied) with 'a' ammount of people '=' "equals" '0' pieces for each person -> a ∈ N because there are no negative people or half ones and lets say i know 100 people then a < 101

in this specific case 0/0 = {0;1;2;...;98;99;100} because I know how many pieces I have and how many pieces everyone will get but I stil don't know how many people will come ... it is actually irrelevant

and last but not least for x/0 and x ≠ 0... this just doesn't exist ... we can imagine it but it simply doesn't exist

it is like a unicorn ... we can't say 100% there is no unicorn (because who knows) but ... if someone asked us if unicorns exist we would tell them "no they don't"

a lot of people can tell you a lot of things ... some say there is a god but others won't believe and we can't prove them wrong ( we cannot prove that 3/0 doesn't exist)

I say in order for things to exist, you need to prove existence with definition and logic... well this is what I believe but maybe im wrong

Mach St
Oct 5, 2017

step 4 has flawed logic, as when working with algebra, ALL TERMS must be divided by the same thing, not just each side for example: 7-7=8-8 7 8 8 \frac{7}{8-8} - 7 8 8 \frac{7}{8-8} = 8 8 8 \frac{8}{8-8} - 8 8 8 \frac{8}{8-8} which works out as 7 0 \frac{7}{0} - 7 0 \frac{7}{0} = 8 0 \frac{8}{0} - 8 0 \frac{8}{0} and as it has been mathematically proven, this is impossible and BADMAS states that multiplication is solved before subtraction, unless brackets state otherwise so 7 7 8 8 \frac{7-7}{8-8} equals 0/0 which is impossible and always ALWAYS explain where numbers come from, like that one for example. why did you chose 1? why not ten? or zero? if you're going to take a number out of context, the entire equation is invalid

thus proving this whole thing false

Locating Wizard
Feb 28, 2019

8 8 8 8 \frac{8-8}{8-8} is equal to 0 0 \frac{0}{0} , not 1. If it was = 1, that would be using the conjecture you are trying to prove in the proof for that conjecture, which doesn't make any sense. So, essentially, this proof comes out to 0 0 \frac{0}{0} = 0 0 \frac{0}{0} , which I suppose is technically true even though both terms are undefined.

That's what I got :)

Lee Sap - 1 year, 6 months ago
Ken Williams
May 4, 2017

Dividing by zero is an illegal action and results in an undefined number tending to infinity - a singularity. Dividing by 8-8 means you are dividing by zero thus the solution is flawed. This concept gives rise to various anomalies in mathematics making things seem something the are not and can be rather quirky. Beware of dividing by zero - try it on your calculator for instance. There are philosophical arguments around this property of zero.

Venkatachalam J
May 2, 2017

Step 4: "Dividing both sides by 8-8" (8-8=0).
Divide by zero is not allowed. By using divide by zero we got the wrong result.

. .
Jul 9, 2020

The answer is Step 4 because 7-7=8-8 and (7-7)/(8-8) is not the same. Proof: 7-7=8-8 equals 0=0 but 0/0 equals any number. I can explain why 0/0 equals any numbers. Prove: if 0/0 is 0, it satisfies 0=0 0 Then 0/0 is 1, it satisfies 0=0 1 And 0/0 is 2, it satisfies 0=0*2 So there are so many answers of 0/0, I explained why 0/0's solution is any numbers. 0/0=x(x is not 0), any value of x satisfies in natural number, integer, rational number, an irrational number, real number, imaginary number, complex number.

Amine Erraqabi
Jan 17, 2019

because of the priority of operations (7-7)x(8-8)^-1 we're supposed to apply the substraction first wich lead us again to 0/0

Marc Moreau
Aug 30, 2020

8-8 equal 0. Deviding by zero is undeterminated, it will be demonstrate.

Odin Wang
Aug 13, 2020

8 does not equal 7, and only equal numbers will cancel out to create 1, so step 4 is incorrect.

Poojan Patel
May 19, 2020

From step 3 to step 4: You said to divide both side by 8-8, so it would represent as 7 7 8 8 \frac{7-7}{8-8} = 8 8 8 8 \frac{8-8}{8-8} . But you considered LHS=1 ------> i.e., 0 0 \frac{0}{0} which you are still proving it as 1. You can't take it as 1. This was the error. Thank You.

Mina Kwack
Apr 27, 2020

0/0=any number since we can't divide normally we can do reverse multiplication so ?x0=0 since anything multiplied by 0=0 in this equation ?=any number which means 0/0=any number

José Santos
Apr 9, 2020

( 8 8 ) ( 8 8 ) \frac{(8-8)}{(8-8)} = 0 0 \frac{0}{0} also ( 7 7 ) ( 8 8 ) \frac{(7-7)}{(8-8)} = 0 0 \frac{0}{0} Therefore, all we can say is that: 0 0 \frac{0}{0} = 0 0 \frac{0}{0}

Marlond Sanchez
Feb 5, 2020

there is no variable that allow an operation can make this problem divide, in another words the variable need to appear in the operation if you want to realize an operation such as multiplication, division, or radication.

Lee Sap
Nov 19, 2019

Step 4 is: 7 7 8 8 \frac{7-7}{8-8} = 8 8 8 8 \frac{8-8}{8-8} . As we already know, 7-7=0 and 8-8=0...... Therefore: 7 7 8 8 \frac{7-7}{8-8} = 0 and 8 8 8 8 \frac{8-8}{8-8} = 0.... So the equation becomes: 0 0 \frac{0}{0} = 0 0 \frac{0}{0} .... So 0 0 \frac{0}{0} does not equal 1 - it equals itself according to this set of equations, and thus, step 4 was incorrect. Step 4 assumes that 8 8 8 8 \frac{8-8}{8-8} is equal to 1, when in fact, it is equal to 0 0 \frac{0}{0} which has not yet been proven to be 1. Hope this makes sense :)

  • Simply because (7-7):(8-8)=(7-7):0 which you're not allowed to
Martin Francák
Jun 3, 2019

From "algorithmic POW" step 4 essentially means "go back to step 1", or create subroutine containing "step 1 to 4", which leads to creating subroutine containing "step 1 to 4", which leads to creating subroutine containing "step 1 to 4", which leads to creating subroutine containing "step 1 to 4"... Poor CPU. As for why 0/0 is sometimes defined as 1 is pratical reasons. Unexpected operations with "1" are not likely to lead to a catastrophe. Unexpected operations with "0" invariably do.

Adelio Muzima
Apr 10, 2019

when you divide the both side of the equation by 8-8, you are assuming that the right side is equal to 1, but 8-8 (zero) divided by 8-8 (zero) is not equal to 1, it is an indeterminacy.

Bob Pop
Apr 9, 2019

I just guessed

David Velasco
Feb 28, 2019

8-8 equals 0.

Carlos Gunner
Feb 8, 2019

As a student of calculus at a university, i say that a division by zero egual= indefinition.

Bing Bong
Dec 12, 2018

step 4. 7-7/8-8 = 0/0 is illegal. this explaination is more like a magic trick designed to lure humans more than math

But why is step 4 illegal?

Pi Han Goh - 2 years, 6 months ago

ITS STEP 3

vina vads - 2 years, 4 months ago
Rain Zhao
Oct 17, 2018

8 8 8 8 \frac{8-8}{8-8} evaluates to 0 0 \frac{0}{0} but we can't just skip that fact and go straight to 1 because that's exactly what we're trying to prove.

Thomas Stedman
Aug 15, 2018

0/0 is 1 because 0/0=0^1/0^1=0^0=1

0/0 = 0^3/0^2 = 0^(3-2) = 0^1 =0 as well. What's wrong with this?

Pi Han Goh - 2 years, 10 months ago
Michael Ryman
Jul 27, 2018

Where zero = 0, logic cannot be made, but where 0 = zero or 0, logic can be assumed because zero and naught are not necessarily the same nor equal.

Bobo Yuan
Jun 8, 2018

In Step 4 you divide by Zero, and its not defined

Aubrey Byrne
May 6, 2018

Step 4, he divided 0 by 0 and got 1 to prove that when you divide 0 by 0 you get 1... circular argument lol

Manthan Singh
Jan 16, 2018

As we divide 8-8\8-8 , so in the denominator we will get 0 as an answer . Therefore, any number divided by 0 will be infinity.

No. Division by 0 is not allowed.

Pi Han Goh - 3 years, 4 months ago
Karthikk Raja
Dec 8, 2017

Dividing 8-8 by 8-8 does not give 1

Alex Wang
Aug 1, 2017

step 4 8-8 is 0 dividing by 0 = ± \pm \infty which is impossible

Munem Shahriar
Jul 28, 2017

Let's check every steps ,

Step:1 We can rewrite 15 15 as 7 + 8 7 + 8 or (8 + 7)

Since 7 + 8 = 15 7 + 8 = 15 or 8 + 7 = 15 8 + 7 = 15 so step 1 is correct.

Step 2: This means that 7 + 8 = 8 + 7 7 + 8 = 8 + 7

15 = 15 15 = 15 so step 2 is correct.

Step 3: If we move one term from each side of the equation to the other side, we will get 7 7 = 8 8 7 - 7 = 8 - 8

Step 3 is correct

Step 4: Dividing from both sides by 8 8 8 - 8 gives 7 7 8 8 = 1 \dfrac{7-7}{8-8} = 1

7 7 8 8 = 0 0 \dfrac{7-7}{8-8} = \dfrac{0}{0} and division by 0 is not allowed , therefore step 4 \boxed{\text{step 4}} is incorrect.

So, do you agree that (8-8)/(8-8)=1 => 0/0=1??? Division by 0??? If you think 0/0 could be 1, than you must agree that (7-7)/(8-8)=1 => 0/0=1 You contradict yourself, and cannot say (7-7)/(8-8) is not equal to 1

Fabiano Barbosa - 3 years, 9 months ago

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