
Casting out nines - fargo
https://en.wikipedia.org/wiki/Casting_out_nines
======
hcase
that is one of the most poorly written wikipedia articles i have ever read

~~~
schoen
Contrast with

[https://en.wikipedia.org/wiki/Digital_root](https://en.wikipedia.org/wiki/Digital_root)

which presents much of the same material and is a decent fully-wikified
article using math notation.

I think the casting out nines article could be improved a lot, maybe with more
of a focus on the history of the concept.

~~~
lgas
I suspect the average non-mathematician would get a lot more out of the
"Casting out nines" page than the "Digital root" page.

~~~
schoen
Sure, it's definitely helpful to have a less-technical article.

------
cproctor
How is this different from "casting out tens," or just checking the accuracy
of the digit in the ones place?

~~~
contravariant
The idea is that you can also just add up the digits of a number to 'cast out
nines' (during this calculation you can also ignore any pair of digits that
adds up to 9).

By the way, you can combine this with 'casting out tens', then you'll know if
your result is accurate mod 90 (which is even less likely than it being
accurate mod 10 or mod 9, if you've made a mistake).

------
fibo
In Italy you learn it at 8 years old. It is also said, "cast out nines"
instead of "double check".

