## AssumptionsEdit

## ProofEdit

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

- .

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

which implies

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