0.9 recurring is 1

0.999… is the same number as 1. It’s counterintuitive, but mathematics says that it’s true. Whether you believe it or not is another matter.

Mathematics and statistics are full of counterintuitive results: common sense tells you something different, and common sense is wrong. One famous example is the Monty Hall problem – I had a long argument with my math teacher and friends about that one!

I’m not even going to try to summarise all of the proofs and arguments for this result. The one I find most convincing is this:

x = 0.999…
10x = 9.999…
10x = 9 + x
9x = 9
x = 1

• 0.999…
• Monty Hall problem