Difference between revisions of "0.999..."
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− | <math>0.999\ldots</math> (or <math>0.\overline{9}</math>) is an equivalent representation of the [[ | + | __TOC__ |
+ | ==Basic Idea== | ||
+ | <math>0.999\ldots</math> (or <math>0.\overline{9}</math>) is an equivalent representation of the [[integer]] <math>1</math>. | ||
It is intuitively clear what the dots after the nines mean. However, to resolve the problem, one needs mathematics beyond the elementary school level math that is needed to understand the question. | It is intuitively clear what the dots after the nines mean. However, to resolve the problem, one needs mathematics beyond the elementary school level math that is needed to understand the question. | ||
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==See Also== | ==See Also== | ||
*[[Geometric sequence]] | *[[Geometric sequence]] | ||
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Latest revision as of 20:04, 4 February 2023
Contents
Basic Idea
(or
) is an equivalent representation of the integer
.
It is intuitively clear what the dots after the nines mean. However, to resolve the problem, one needs mathematics beyond the elementary school level math that is needed to understand the question.
What do the dots after the s actually mean?
One potential definition is to say that is the limit of the sequence
. That is to say,
is
where there are
nines. Then we can say that
does in fact equal
.
But in some senses, this is not the most satisfying definition. When we say that the limit of as
goes to infinity is equal to
, that means that for all
, there exists an integer
such that
for all integers greater than
. So in fact, we had to resort to limit definitions to resolve this problem. If you think about it, there is no
such that
.
Another mathematical tidbit: how do we know that is a real number? Intuitively, a real number line seems to be continuous, so such a limit seems to be a real number. But how do we know that there are no 'gaps' so it could possibly not be a real number. But lets prove it.
Let .
, so
is nonempty. It is easy to see that
since it is a finite geometric series. But then
. Hence
has an upper bound of
. But by the least upper bound axiom of real numbers, if a set of real numbers has a upper bound, then it has a least upper bound (that is a real number).
Suppose that had a least upper bound,
such that
. But then we have
. But that is nonsense since
, which would be a least upper bound for the natural numbers. Hence
, the limit of the sequence, is a least upper bound. Hence that expression is in fact a real number.
The proofs below show how to evaluate the limit, but they implicitly use the definition given above.
Proofs
Fractions
Since , multiplying both sides by
yields
Alternatively, , and then multiply both sides by
.
Algebraic Manipulation
Let Then we have
Subtracting,
Infinite series
![$0.999\ldots = 0.9 + 0.09 + 0.009 + \ldots = \frac{9}{10} + \frac{9}{100} + \frac{9}{1000} + \ldots$](http://latex.artofproblemsolving.com/c/0/7/c0790728328bd92ec568c11fd893412ba7ee7aba.png)
This is an infinite geometric series, so
![$0.999\ldots = \frac{\frac{9}{10}}{1 - \frac{1}{10}} = 1$](http://latex.artofproblemsolving.com/a/e/2/ae2f54a6432e25455253212354eaa9db3238648c.png)
Limits