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And 299999999 is divisible by 13 (lemmy.world)

submitted 2 weeks ago by [email protected] to c/[email protected]

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[–] [email protected] 87 points 2 weeks 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] 63 points 2 weeks ago (1 children)

Thanks I hate it

[–] [email protected] 9 points 2 weeks ago

This math will not stand man!

[–] [email protected] 13 points 2 weeks 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 2 weeks 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 2 weeks ago* (last edited 2 weeks 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.

[–] JamesStallion 2 points 2 weeks ago

Fucking lol

[–] Scubus 3 points 2 weeks ago (1 children)

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

[–] [email protected] 3 points 2 weeks ago (1 children)

Never say never. 1/0 = 0

[–] [email protected] 1 points 2 weeks ago

Interesting read. Thank you.

[–] [email protected] 1 points 2 weeks ago

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

[–] [email protected] 5 points 2 weeks ago

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