# "0.99999999... = 1 ?"의 두 판 사이의 차이

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0.9999… = 1 이다.

## 노트

### 말뭉치

1. One might argue that when multiplying by ten, the right hand side shifts to the left a decimal place, leaving a terminating zero at the end of the infinite string of 9's ( ).[1]
2. And, therefore, when subtracting (with a one after the last of infinite nines).[1]
3. It is impossible to find a "next higher" number that is both larger to a given value but couldnt have been smaller (see argument from averages).[1]
4. One might argue that after the infinitely many zeros, there is going to be a 1 ( ).[1]
5. The sequence S = (0.9, 0.99, 0.999, … ) is bounded above by 1.0, so the least upper bound is not greater than 1.0.[2]
6. If we consider the surreal number 0.999… (see earlier post), we may assume that there are ω 9s after the decimal.[2]
7. Return to the Lessons Index | Do the Lessons in Order | Print-friendly page How Can 0.999... = 1?[3]
8. Whatever the reason, many students (me included) have, at one time or another, felt uncomfortable with this equality.[3]
9. The ellipsis (the "dot, dot, dot" after the last 9 ) means "goes on forever in like manner".[3]
10. Yes, at any given stop, at any given stage of the expansion, for any given finite number of 9 s, there will be a difference between 0.999...9 and 1 .[3]
11. In mathematics, 0.999... (also written as 0.9, in repeating decimal notation) denotes the repeating decimal consisting of infinitely many 9s after the decimal point.[4]
12. In other words, "0.999..." and "1" represent the same number.[4]
13. The technique used depends on the target audience, background assumptions, historical context, and preferred development of the real numbers, the system within which 0.999... is commonly defined.[4]
14. More generally, every nonzero terminating decimal has two equal representations (for example, 8.32 and 8.31999...), which is a property of all base representations.[4]
15. So when we learn math in elementary school we are told: Every real number can be written as a decimal expansion (maybe infinite) and every possible decimal expansion is a real number.[5]
16. we take it on faith that: Every real number can be written as a decimal expansion (maybe infinite) and every possible decimal expansion is a real number.[5]
17. (So it can be all the elements less than 1/2 but it can't be all the elements less than or equal to 1/2).[5]
18. Notice sometimes the cut will occur at a rational number (all the rationals less than 1/2), but sometimes it will occur at points "between" the rational numbers.[5]
19. 다행히도, 임의의 무한소수에 대해 수열의 극한값은 존재하고, 그 극한값은 이 수열의 상한(supremum), 풀어 쓰면 '모든 자연수에 대해보다 크거나 같은 숫자의 집합에서 가장 작은 수' 와 같다.이를 증명하기는 어렵지 않다.[6]
20. IF the two numbers 0.99999... and 1 were NOT equal, you would have a number to represent the difference (the GAP or Distance between them) - and it should not be 0.[7]
21. Of course it is a bit obscure since we are dealing with 'infinite processes' (sometimes called limits) whenever we work with decimal numbers which do no terminate.[7]
22. If you stop the expansion of 9s at any finite point, the fraction you have (like .99999 = 99999/100000) is never equal to 1.[8]
23. But each time you add a 9, the margin error is smaller (with each 9, the error is actually ten times smaller).[8]
24. You can show (using calculus or other summations) that with a large enough number of 9s in the expansion, you can get arbitrarily close to 1.[8]
25. To prove it to yourself that 0.9999… = 1, consider that if they weren’t equal, there would be a number E that is greater than zero such that E = (1 — 0.9999…).[9]
26. Since we’re playing this game, you counter and make E smaller, say 10^(-10), and I turn around and say “make D = 11” (because 1 — 0.99999999999 < 10^(-10) ).[9]
27. Specifically, if E > 10^(-X) for some positive integer X, then setting D = X will do it.[9]
28. There is no E that is greater than zero such that E = (1 — 0.9999…).[9]
29. Well, it’s “true” in the same way numbers must be there (positive) or not there (zero) — it’s true because we’ve implicitly excluded other possibilities.[10]
30. The idea behind limits is to find some point at which “a – b” becomes zero (less than any number); that is, we can’t tell the “number to test” and our “result” as different.[10]
31. It’s still tough to compare items when they take such different forms (like an infinite series).[10]
32. Then if you ask for 1, and I give you 0.99, the difference is 0.01 (one hundredth) and you don’t know you’ve been tricked![10]
33. Student: Are you (looks at Vadim) saying that you (looks at Teacher) made up the definition of a limit just in order to make 0.999…=1?[11]
34. 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).[12]
35. That must mean that one-third of it ("point 3 repeating") is also irrational.[13]
36. And whenever you have an infinite number of parts of something (an infinite number of places in a number, in this case) you might well expect something amazing to happen.[14]
37. Said another way, if your number x = 0.999999999... is such that the difference "x-1" is smaller than any positive number (however small), then we consider x - 1 = 0, so that x = 1.[14]
38. If we do this, we'll get a 9.999990 (slide the decimal in the 0.999999 right 1 places, you'll get 9.999990).[15]
39. If you can warranted that you always do the best on your job level, your boss will automatically think that you must get (absolutely get) new level on your job.[16]
40. The standard algebra proof (which, if you modify it a little, works to convert any repeating decimal into a fraction) runs something like this.[17]
41. Those are my first two demonstrations that our fact is true (the last one is at the end).[17]
42. The word "geometric" means that each term of the series is the identical multiple (in this case 1/10) of the previous term.[17]
43. You’ll see the same kind of thing in all languages that support your hardware’s floating-point arithmetic (although some languages may not display the difference by default, or in all output modes).[18]
44. The documentation for the built-in round() function says that it rounds to the nearest value, rounding ties away from zero.[18]
45. Since the decimal fraction 2.675 is exactly halfway between 2.67 and 2.68, you might expect the result here to be (a binary approximation to) 2.68.[18]
46. For fine control over how a float is displayed see the str.format() method’s format specifiers in Format String Syntax.[18]
47. If you had a hat with an infinite amount of slips of paper consisting of an infinite amount of numbers, the probability of pulling any random number is zero (1/infinity = 0).[19]
48. In fact, the probability of pulling any real subset of numbers is zero (i.e. 10000/infinity = 0).[19]

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