Poll: 0.999... = 1

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Nov 24, 2010
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zfactor said:
Back on topic, .999999999999999999999999999999999999999999999999999999999999999999999999999(and so on) does not equal 1 because 1 is 1.0000000000000000 not (above .999...). They are two different numbers which can be rounded to each other, but are not equal to each other.
You misunderstand limits.
 

SL33TBL1ND

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Nov 9, 2008
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Since the difference between 0.9... and 1 is so infinitely minuscule, it might as well be 1.
 

Rubashov

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Jun 23, 2010
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zfactor said:
havass said:
If x = 0.999999...
Then 10x = 9.9999...
Therefore, 10x - x = 9
Which implies 9x = 9
Thus, x = 1
x also = 0.99999...

In conclusion, I have just proven 1 = 0.9999...
Uh, wait, the second part will have one less 9 after the decimal point than the first part.
No. That's not how infinity works. Both 10x and x have infinite decimal places, so it doesn't make sense to talk about one having one less decimal place than the other.
 

zfactor

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Jan 16, 2010
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crudus said:
Sturmdolch said:
havass said:
If x = 0.999999...
Then 10x = 9.9999...
Therefore, 10x - x = 9
Which implies 9x = 9
Thus, x = 1
x also = 0.99999...

In conclusion, I have just proven 1 = 0.9999...
You're doing it wrong, too. At step 4, 9x = 9, that is not true.

9x = 8.99999999
I think you are going to have to explain that one.
x = 0.9999999999 (a finite number of 9s, for proof of concept purposes [or you can use infinity])
10x = 9.999999999 (this is one less 9 than the number of 9s in x [or infinity - 1])

So the 10x - x would actually equal 8.9999999991, not 9. [or 8.(infinity - 1 nines)1] And no, that fraction shouldn't theoretically explode because the extra 1 on the end is the decimal place provided by subtracting 1 from infinity (providing you can do that, and in my brain, yes you can).
 

crudus

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Oct 20, 2008
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BlacklightVirus said:
To quote a math major who is a friend of mine:

I'm letting a_n = .99...9, with n 9s. So a_1 = .9, a_2=.99, etc.

Consider the closed interval [a_n, 1]. I want to show that .999...=1, and so I'm going to show that as n goes to infinity, there is exactly one number in this set. Since 1 is obviously in it, and .999...= a_n as n goes to infinity, this will imply the equality.

Note that for n < m, a_n < a_m. So, a_m is in [a_n, 1]. It's clear then that [a_n, 1] as n goes to infinity is the same thing as the intersection over all n of these closed intervals. That is, I'm going to only look at numbers that are in every such closed interval.

Set A to be equal to the set of all numbers in every such interval. Since 1 is in every interval by definition, A is not empty. If A has more than one point, then the diameter of A is larger than 0. But a_n --> 1 as n--> infinity, and diameter of A is smaller than the diameter of [a_n, 1].

So, we end up with the inequality 0 < diam(A) < diam( [a_n,1] ). The left and right sides go to 0, implying that diam(A) does as well. This contradicts the assumption that two points are in A.

But note that for any n, .999... > a_n, and so .999... must be in A as well (since it's in every interval). Thus, .999...=1.
Wow, that was surprisingly hard to wrap my head around.

enriel said:
.9(infinite) = 1 because my brain can't reasonably comprehend the difference between the two numbers.

This isn't a math puzzle as much as a logic puzzle. What's being invoked is a Zeno's Paradox. .9(infinite) gets as close as possible to 1 without ever actually being 1.

Therefore, technically, no, they are not the same number. However, for simplicity's sake they may as well be, since nobody can really understand an infinitesimally small difference.
except Zeno didn't know what a limit was.
 

digits

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Aug 5, 2009
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This is true, in the same way that 1 divided by zero is infinity. And for the geometric proof. That is the number it converges to, not the number it equals. Therefor, it is still not very convincing. You misunderstood the meaning of convergence.
 

zfactor

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Rubashov said:
zfactor said:
havass said:
If x = 0.999999...
Then 10x = 9.9999...
Therefore, 10x - x = 9
Which implies 9x = 9
Thus, x = 1
x also = 0.99999...

In conclusion, I have just proven 1 = 0.9999...
Uh, wait, the second part will have one less 9 after the decimal point than the first part.
No. That's not how infinity works. Both 10x and x have infinite decimal places, so it doesn't make sense to talk about one having one less decimal place than the other.
He multiplied by 10 thus shifting the decimal place over to the right one space. X had infinity decimal places and 10x has infinity - 1 decimal places. The infinite decimal places applies to x, not to the polynomial 10x. At least that's how I see it, the answer to the entire question relies on how you work with infinity...
 

waxwingslain

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Aug 12, 2009
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How about this one:

The difference between 0.9999.... and 1.0 is 0.0000... and since the difference between them is 0, the two numbers must be equal.
 

Bon_Clay

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Well your poll is broken, but if it weren't I would point out that math isn't about opinions. The concept of infinity can get complex sometimes in math, it doesn't always mean exactly the same thing. If there's a proof for it, and it doesn't violate any rules like that one on the t-shirt on the first page, then that's the way it is.
 

enriel

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crudus said:
enriel said:
.9(infinite) = 1 because my brain can't reasonably comprehend the difference between the two numbers.

This isn't a math puzzle as much as a logic puzzle. What's being invoked is a Zeno's Paradox. .9(infinite) gets as close as possible to 1 without ever actually being 1.

Therefore, technically, no, they are not the same number. However, for simplicity's sake they may as well be, since nobody can really understand an infinitesimally small difference.
except Zeno didn't know what a limit was.
True enough, but I still maintain that regardless of mathematical nonsense, this stands as a logic puzzle. .9(infinite) and 1 are not the same number because they are notated differently and therefore must be different, no matter how immeasurable that amount may be. But they can be treated as the same number BECAUSE that amount is so tiny, it wouldn't affect anything in practical use.
 

crudus

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zfactor said:
x = 0.9999999999 (a finite number of 9s, for proof of concept purposes [or you can use infinity])
10x = 9.999999999 (this is one less 9 than the number of 9s in x [or infinity - 1])

So the 10x - x would actually equal 8.9999999991, not 9. [or 8.(infinity - 1 nines)1] And no, that fraction shouldn't theoretically explode because the extra 1 on the end is the decimal place provided by subtracting 1 from infinity (providing you can do that, and in my brain, yes you can).
Except you really aren't taking into account the <a href=http://en.wikipedia.org/wiki/Hilbert%27s_paradox_of_the_Grand_Hotel>Hilbert's Hotel concept that the proof relies on. Although, both point is moot. The proof seems to rely on the proof being true to be true.
 

Rabid Toilet

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Mar 23, 2008
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Oh god, not this thread again.

Yes, it's fun to boggle the minds of people who don't understand higher math, but do we really need to do this again? It's just going to end up as another 100 page thread of people who don't understand the concept arguing with the people who do, and nothing is ever resolved.
 

CrystalShadow

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Apr 11, 2009
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crudus said:
Naheal said:
Actually, it didn't :-/ That proof works.
It does. Between step 5 and step 6 it divides both sides by (a-b). a=b therefore (a-b)=0.

BlacklightVirus said:
[There is an even simpler proof.

1/3 = 0.333...

1/3 + 1/3 + 1/3 = 3/3 = 1

But 0.333... + 0.333... + 0.333 = 0.999...

Hence 0.999 = 1
If I remember right you are almost begging the question there. You are using the concept to prove itself. Again, don't remember the exact reasoning behind it. So far the most satisfying reason I have seen was the infinite series proof.
But isn't math begging the question by default?

Eh. Maybe not. But for all it's inherent logic, Math in the end still comes down to it's axioms. And those axioms are completely arbitrary.

They have to be, because if you can logically derive them, then you can decompose them into a combination of other parts that logically lead to them.

Therefore, the most fundamental axioms cannot themselves be logical statements, or you would have a infinite regression.

Fundamental problem with logic all round to be honest.

Logic, fundamentally, isn't logical. XD
 

emeraldrafael

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havass said:
If x = 0.999999...
Then 10x = 9.9999...
Therefore, 10x - x = 9
Which implies 9x = 9
Thus, x = 1
x also = 0.99999...

In conclusion, I have just proven 1 = 0.9999...
I want to go with this, cause I dont know how i would disprove it.

But I would say no unless it says round up, simply cause no two numbers can be the same.
 

Rubashov

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Jun 23, 2010
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digits said:
This is true, in the same way that 1 divided by zero is infinity. And for the geometric proof. That is the number it converges to, not the number it equals. Therefore, it is still not very convincing. You misunderstood the meaning of convergence.
No. The sum of an infinite series is defined as the limit of the sequence of partial sums. If the limit of the sequence of partial sums converges, then the infinite series equals the value to which that limit converges.
 

Rabid Toilet

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enriel said:
.9(infinite) and 1 are not the same number because they are notated differently and therefore must be different, no matter how immeasurable that amount may be. But they can be treated as the same number BECAUSE that amount is so tiny, it wouldn't affect anything in practical use.
God damnit, I'm getting sucked into this, but I can't let this one slide. Two numbers that are notated differently are not automatically different numbers.

3/6 = .5

Same. Number.

.999... = 3/3 = 1

Still. Same. Number.
 

CrystalShadow

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Apr 11, 2009
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zfactor said:
Rubashov said:
zfactor said:
havass said:
If x = 0.999999...
Then 10x = 9.9999...
Therefore, 10x - x = 9
Which implies 9x = 9
Thus, x = 1
x also = 0.99999...

In conclusion, I have just proven 1 = 0.9999...
Uh, wait, the second part will have one less 9 after the decimal point than the first part.
No. That's not how infinity works. Both 10x and x have infinite decimal places, so it doesn't make sense to talk about one having one less decimal place than the other.
He multiplied by 10 thus shifting the decimal place over to the right one space. X had infinity decimal places and 10x has infinity - 1 decimal places. The infinite decimal places applies to x, not to the polynomial 10x. At least that's how I see it, the answer to the entire question relies on how you work with infinity...
But... Isn't infinity - 1 = infinity?

It's not even a different class of infinity, so that doesn't really help any.

Whenever infinity is invoked in anything, the results are difficult to interpret.

Suffice to say, if: infinity - 1 = infinity, then x and 10x in this case both have the same number of decimal places.

It's only if: infinity - 1 =/= infinity, that the example holds as it's being explained.
 

zfactor

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Jan 16, 2010
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BlacklightVirus said:
zfactor said:
Back on topic, .999999999999999999999999999999999999999999999999999999999999999999999999999(and so on) does not equal 1 because 1 is 1.0000000000000000 not (above .999...). They are two different numbers which can be rounded to each other, but are not equal to each other.
You misunderstand limits.
Erm, what?

I was saying .9999[infinity] is not 1 because they are not the same number: one is made up of bagillions of 9s and the other is a 1. Only same numbers can be set equal to each other, they are not the same number, they cannot be set equal to each other.

I was trying to use logic and avoid math... -sigh-

Let's say a_1 is .9; a_2 is .99; a_3 is .999; ... ; a_x is .[x nines].

The limit of a_x as x => infinity is, indeed, 1. However, a_[infinty] does not equal 1. The value is still .9999[infinity]. a_x will never equal 1. You can round it to equal 1, but it still will never be exactly 1...
 

Techmech

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Sep 29, 2010
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This is actually a question related to calculus.
You'd be better off not describing X as X=0.9999999999999...
but rather X -> 1
which is to say, X approaches infinatly close to the value of 1, without ever actually becoming 1.

Treated algebraically, your proof is indeed correct, But as we can see, it ultimately fails the sanity test. Saying 0.999999999999=1 is ridiculous.

So in order to truly deal with the question, one requires a working knowledge of the fundamental principals of calculus.