Why does 1 have two expression forms in decimal 1 and 0.999......?

52 Views Asked by At

1 has two expression forms in decimal 1 and 0.999......, but others do not have only one expression form. Why?

1

There are 1 best solutions below

0
On BEST ANSWER

Other numbers do have multiple decimal expressions. E.g. $0.43 = 0.42999 \dots$. In general, any number that ends in a string of 0s will have two ways of writing it.

Because it ends in a string of 0s, any change in either direction won't just change the digits in the 1000th place or whatever, but will change the 2nd decimal place of $0.43$. This makes it possible to get closer and closer to $0.43$ with a 2 or a 3 in the second decimal place.

For any number not ending in a string of 0s, to get closer and closer, you'll have to agree with the number on more and more decimal places. E.g. you can't get within $0.001$ of $0.333 \dots$ without having 3s in the first and second place. You can't get within $10^{-n}$ of it without having a 3 in the $n-1$th place.

The fact that you have to agree on more digits to get closer means that for any decimal to represent it exactly, it must agree on every digit. That's why numbers not ending in a string of 0s can only have one decimal representation.