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And 299999999 is divisible by 13 (lemmy.world)

submitted 3 days ago by [email protected] to c/[email protected]

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[–] [email protected] 86 points 3 days ago (3 children)

The divisability rule for 7 is that the difference of doubled last digit of a number and the remaining part of that number is divisible by 7.

E.g. 299'999 → 29'999 - 18 = 29'981 → 2'998 - 2 = 2'996 → 299 - 12 = 287 → 28 - 14 = 14 → 14 mod 7 = 0.

It's a very nasty divisibility rule. The one for 13 works in the same way, but instead of multiplying by 2, you multiply by 4. There are actually a couple of well-known rules for that, but these are the easiest to remember IMO.

[–] [email protected] 62 points 3 days ago (1 children)

Thanks I hate it

[–] [email protected] 9 points 3 days ago

This math will not stand man!

[–] [email protected] 13 points 3 days ago (2 children)

If all of the digits summed recursively reduce to a 9, then the number is divisible by 9 and also by 3.

If the difference between the sums of alternating sets digits in a number is divisible by 11, then the number itself is divisible by 11.

That’s all I can remember, but yay for math right?

[–] [email protected] 8 points 3 days ago (2 children)

Well, on the side of easy ones there is "if the last digit is divisible by 2, whole number is divisible by 2". Also works for 5. And if you take last 2 digits, it works for 4. And the legendary "if it ends with 0, it's divisible by 10".

[–] [email protected] 9 points 3 days ago* (last edited 3 days ago) (1 children)

There’s also the classic “no three positive integers a, b, and c to satisfy a**n + b**n = c**n for values of n greater than 2“ trick but my proof is too large to fit in this comment.

[–] [email protected] 2 points 3 days ago

Fucking lol

[–] [email protected] 3 points 3 days ago (1 children)

Its never divisible by zero, and its always divisible by one

[–] [email protected] 3 points 3 days ago (1 children)

Never say never. 1/0 = 0

[–] [email protected] 1 points 3 days ago

Interesting read. Thank you.

[–] [email protected] 1 points 3 days ago

The 9 rule works for 3 too The 6 rule is if (divisible by 3 and divisible by 2)

[–] [email protected] 5 points 3 days ago

I think it might be easier just to do the division.