# 0.999.../Proof by equality of Dedekind cuts

## Proof

In the Dedekind cut approach, the real number 1 is the set of all rational numbers that are less than 1. Meanwhile, the real number 0.999... is the set of rational numbers r such that r < 0, or r < 0.9, or r < 0.99, or r is less than some other number of the form

{\begin{aligned}1-\left({\tfrac {1}{10}}\right)^{n}\end{aligned}}.

Every element of 0.999... is less than 1, so it is an element of the real number 1. Conversely, an element of 1 is a rational number

{\begin{aligned}{\tfrac {a}{b}}<1\end{aligned}},

which implies

{\begin{aligned}{\tfrac {a}{b}}<1-\left({\tfrac {1}{10}}\right)^{b}\end{aligned}}.

Since 0.999... and 1 contain the same rational numbers, they are the same set: 0.999... = 1.