this post was submitted on 04 Nov 2024
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And 299999999 is divisible by 13 (lemmy.world)
submitted 2 weeks ago by m4m4m4m4@lemmy.world to c/science_memes@mander.xyz
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[–] TwilightKiddy@programming.dev 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.

[–] urda@lebowski.social 63 points 2 weeks ago (1 children)

Thanks I hate it

[–] veroxii@aussie.zone 9 points 2 weeks ago

This math will not stand man!

[–] darkpanda@lemmy.ca 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?

[–] TwilightKiddy@programming.dev 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".

[–] darkpanda@lemmy.ca 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.

[–] Scubus@sh.itjust.works 3 points 2 weeks ago (1 children)

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

[–] wicked@programming.dev 3 points 2 weeks ago (1 children)

Never say never. 1/0 = 0

[–] LordTrychon@startrek.website 1 points 2 weeks ago

Interesting read. Thank you.

[–] levzzz@lemmy.world 1 points 2 weeks ago

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

[–] candybrie@lemmy.world 5 points 2 weeks ago

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