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Casting out nines
- fibo 11y agoIn Italy you learn it at 8 years old. It is also said, "cast out nines" instead of "double check".
- hcase 11y agothat is one of the most poorly written wikipedia articles i have ever read
- schoen 11y agoContrast 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.
- shawkinaw 11y agoSeriously ... I had to go all the way to "Further Examples" just to get any idea what it was talking about.
- cproctor 11y agoHow is this different from "casting out tens," or just checking the accuracy of the digit in the ones place?
- dkbrk 11y agoThis method uses all the digits in the number so that it is possible to detect an error in any position, not just in the least-significant digit.
- contravariant 11y agoThe 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).