Friday, October 5, 2018

real analysis - Is it true that $0.999999999dots=1$?




I'm told by smart people that
$$0.999999999\dots=1$$
and I believe them, but is there a proof that explains why this is?


Answer



What does it mean when you refer to $.99999\ldots$? Symbols don't mean anything in particular until you've defined what you mean by them.



In this case the definition is that you are taking the limit of $.9$, $.99$, $.999$, $.9999$, etc. What does it mean to say that limit is $1$? Well, it means that no matter how small a number $x$ you pick, I can show you a point in that sequence such that all further numbers in the sequence are within distance $x$ of $1$. But certainly whatever number you choose your number is bigger than $10^{-k}$ for some $k$. So I can just pick my point to be the $k$th spot in the sequence.



A more intuitive way of explaining the above argument is that the reason $.99999\ldots = 1$ is that their difference is zero. So let's subtract $1.0000\ldots -.99999\ldots = .00000\ldots = 0$. That is,




$1.0 -.9 = .1$



$1.00-.99 = .01$



$1.000-.999=.001$,



$\ldots$



$1.000\ldots -.99999\ldots = .000\ldots = 0$



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