Question 1207933
.


It is discussed and explained in all details in this Wikipedia article


<A HREF=https://en.wikipedia.org/wiki/0.999...>https://en.wikipedia.org/wiki/0.999...</A>


https://en.wikipedia.org/wiki/0.999...



It is also explained in every fundamental course / (textbook)  of Calculus at the very beginning,
when the theory of real numbers is explained.



So, there is nothing mysterious in it.  It is a routine fact of the theory or real numbers.