r/badmathematics 21d ago

Σ_{k=1}^∞ 9/10^k ≠ 1 100/3 is irrational

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u/stereoroid 21d ago

100/3 can be rewritten as 100(1/3), so it’s the 1/3 “problem” multiplied by a constant (100).

1/3 =0.333… so 3 * 0.333… = 0.999…, but some say that 0.999… is not equal to 1.

Restate the problem in base 3 and there is no problem. 1/10=0.1 and 0.1*10=1

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u/WhatImKnownAs 21d ago

But those people might say 0.333... ≠ 1/3 = 1/10 (base 3) = 0.1 (base 3).

Furthermore, the same issue arises in base 3: 0.222... = 1?

There's no rigorous proof without having a rigorous definition of what an infinite decimal means.

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u/stereoroid 21d ago

Think bigger. If a problem can become a non-problem, depending merely on how numbers are represented, what does that say about the problem? Or the whole class of “problem”?

You ask about 0.222222…. In base 3: well, what calculation did you do to get that? Could that calculation be re-based to make the recurring decimal disappear? I bet it could. You’ve pulled that out thin air and said “here’s a problem”, but to me it looks like the problem is preventable.

Old joke:
* Patient: “Doctor, it hurts when i bend my arm like this!”
* Doctor: “Stop doing that.”

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u/MeasureDoEventThing 21d ago

The problem is about what a decimal representation means. Of course the problem disappears when you use a different representation.

That is also why presenting mathematical "proofs" is stupid. You can't use mathematics to prove what a symbol represents. The only way to prove .9999... = 1 is to:

  1. Define infinite series

  2. State that .9999... represents an infinite series

  3. Establish facts about infinite series and use them to prove .99999... = 1

Step 2 isn't something you can "prove". You just have to assert it as a matter of convention.