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-   -   0.9 repeating = 1? (http://www.chiefdelphi.com/forums/showthread.php?t=81285)

jmanela 27-01-2010 17:59

0.9 repeating = 1?
 
I was googling around today and i came across something rather odd. http://en.wikipedia.org/wiki/0.999.... Now, this wikipidia page claims that .99999 repeating = 1.

Here is a quick algebraic proof to support this,

Quote:

1/3 = 0.3~ \\common knowledge, let (~ stand for repeating)

(3)1/3 = 0.3~(3) \\multiply 3 to both sides

3/3 = 0.9~ \\evaluate above

1 = 0.9~ \\simplify and voila!
Now that is an algebraic point of view. I can't speak for calculus because i haven't taken that yet. What is your view on this topic?

kwotremb 27-01-2010 18:40

Re: 0.9 repeating = 1?
 
What is the difference between .9 repeating and 1?

Hint: a point.

Now whats the dimension of a point?

Cory 27-01-2010 19:09

Re: 0.9 repeating = 1?
 
When you get to calculus they'll have you prove that .9999=1 using geometric series.

JewishDan18 27-01-2010 21:15

Re: 0.9 repeating = 1?
 
I like this method. between any 2 different real numbers, there is at least one number between them not equal to either of them.

Ian Curtis 27-01-2010 21:21

Re: 0.9 repeating = 1?
 
Quote:

Originally Posted by jmanela (Post 908042)
I was googling around today and i came across something rather odd. http://en.wikipedia.org/wiki/0.999.... Now, this wikipidia page claims that .99999 repeating = 1.

Here is a quick algebraic proof to support this,



Now that is an algebraic point of view. I can't speak for calculus because i haven't taken that yet. What is your view on this topic?

You don't need calculus for this, just Series and Sequences which is an Algebra II/Precalc class. (You will make extensive use of series and sequences in Calc II and you will learn to hate them :rolleyes:)

http://en.wikipedia.org/wiki/Geometric_series

Consider .9~ to have a seed value of .9 with a ratio of .1. Use the formula proven on the wiki page. Voila!

They also do an example problem under Repeated Decimals. :)

Andrew.Jensen 27-01-2010 21:28

Re: 0.9 repeating = 1?
 
I actually figured out that same proof in the Wikipedia article earlier this school year before reading about it. But 0.99 repeating does equal one. There is enough proof, and even common sense, as 1-0.999... =0.000... which is just 0.

DonRotolo 27-01-2010 21:37

Re: 0.9 repeating = 1?
 
You'd be surprised how many people argue that "it can't be true". But it is. The key word is "repeating".

Now for the extra credit: What's the name of that line that goes over the last digit to indicate that it repeats infinitely?

Spoiler for The Answer:

Ian Curtis 27-01-2010 21:52

Re: 0.9 repeating = 1?
 
Quote:

Originally Posted by Andrew.Jensen (Post 908177)
I actually figured out that same proof in the Wikipedia article earlier this school year before reading about it. But 0.99 repeating does equal one. There is enough proof, and even common sense, as 1-0.999... =0.000... which is just 0.

I'm no math major (though I did make the mistake of taking a proof based Linear Algebra class once, never doing that again! :ahh:), but I don't really see the proof. Wouldn't accepting that 0.000... is equal to zero hinge on accepting that 0.999... is equal to one? What am I missing? :o

Mr. Lim 27-01-2010 22:18

Re: 0.9 repeating = 1?
 
Then I guess this means that (lim x-> 1-) is NOT 0.999~ :p

EricH 28-01-2010 01:08

Re: 0.9 repeating = 1?
 
Quote:

Originally Posted by Mr. Lim (Post 908224)
Then I guess this means that (lim x-> 1-) is NOT 0.999~ :p

You mean (lim x-> 1-) of x is not 0.999~, don't you? As it is, you aren't taking a limit at all.;)


By the way, after a certain point, it won't matter; your machinist won't go to that tight of a tolerance...:p

DjMaddius 29-01-2010 07:02

Re: 0.9 repeating = 1?
 
Your wrong. 1/3 = 0.33333333333333333. an infinite amount of 3's. Not 0.33. So multiple .33333~*3 = .99999~, Not 1.

Dad1279 06-02-2010 13:42

Re: 0.9 repeating = 1?
 
Quote:

Originally Posted by DjMaddius (Post 909132)
Your wrong. 1/3 = 0.33333333333333333. an infinite amount of 3's. Not 0.33. So multiple .33333~*3 = .99999~, Not 1.

(1/3) * 3 = 1 = .33333~*3 = .99999~

AndrewJS 13-03-2010 10:35

Re: 0.9 repeating = 1?
 
Just for curiosity, what would happen if you approached the problem backwards, starting at 1, then subtracting:
1 - .1 = .9
1 - .01 = .99
1 -.001 = .999
...
1 - 10^-n = .9~ as n approaches infinity

You could argue the 10^(-infinity) approaches 0 and thus 1 = .9~
But then again, the equivalent expression, 1/10^n divides the number 1 into smaller and smaller parts. Just because the parts are smaller doesn't mean they are nonexistent. To me, this expression states that you can always divide 1 into smaller parts, therefore you can never really "reach" infinity, and therefore never "reach" .9~

It's just like 1/x . What happens as x --> infinity? "The limit is 0." Yes, but does the function itself every EQUAL 0? It gets closer to it, and closer, and closer, .01 .001 .00001 .00000000000000000000001 But there's always that 1 at the end, no matter how many zeros you throw in there. And you can't really put in "an infinite" amount of zeros in between, because then the 1 would be coming after "infinity" which nulls its definition.

I may be wrong, but at least presents an argument different from the standard 1/3 stuff.

Molten 13-03-2010 11:14

Re: 0.9 repeating = 1?
 
I agree that .9~=1 if .3~=1/3. I just think it'd be more accurate to say that both are extremely close approximations. Though most mathemeticians will disagree with me.

Ether 23-08-2010 09:36

Re: 0.9 repeating = 1?
 
Quote:

Originally Posted by JewishDan18 (Post 908167)
I like this method. between any 2 different real numbers, there is at least one number between them not equal to either of them.

between any 2 different real numbers, there exists an uncountably infinite set of different real numbers.


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