Panik

Comments

• logicbomb@lemmy.world ⁨1⁩ ⁨year⁩ ago

  Also, any number whose digits sum to a multiple of 3 is divisible by 3. For 51, 5+1=6, and 6 is a multiple of 3, so 51 can be cleanly divided by 3.

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  • Vorticity@lemmy.world ⁨1⁩ ⁨year⁩ ago

    I’d forgotten this trick. It works for large numbers too. 122,300,223÷3=4,07

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    • FlexibleToast@lemmy.world ⁨1⁩ ⁨year⁩ ago

      The neat part is that if you add the numbers together and they’re still too large to tell, you can do it again. In your example, you get 15. If you do it again, you get 6, which isn’t the best example because 15 is pretty obvious, but it works.

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  • paddirn@lemmy.world ⁨1⁩ ⁨year⁩ ago

    Witchcraft! Burn them!

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    • Steeve@lemmy.ca ⁨1⁩ ⁨year⁩ ago

      She turned me into a newt!

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  • MechanicalJester@lemm.ee ⁨1⁩ ⁨year⁩ ago

    Fuck you and take an upvote for coming here to state what I was going to when I immediately summed 5+1 to 6 and felt clever thinking “well I do know it’s not prime and divisible by 3” Shakes fist

    I’ll get you NEXT time logicbomb!

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    • Dagwood222@lemm.ee ⁨1⁩ ⁨year⁩ ago

      Posted the same info. Silly me

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  • beckerist@lemmy.world ⁨1⁩ ⁨year⁩ ago

    Same with 9. There are rules for every number at least through 13 that I once knew…

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    • logicbomb@lemmy.world ⁨1⁩ ⁨year⁩ ago

      I only know rules for 2 (even number), 3 (digits sum to 3), 4 (last two digits are divisible by 4), 5 (ends in 5 or 0), 6 (if it satisfies the rules for both 3 and 2), 9 (digits sum to 9), and 10 (ends in 0).

      I don’t know of one for 7, 8 or 13. 11 has a limited goofy one that involves seeing if the outer digits sum to the inner digits. 12 is divisible by both 3 and 4, so like 6, it has to satisfy both of those rules.

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  • kibiz0r@midwest.social ⁨1⁩ ⁨year⁩ ago

    What does the proof for this look like?

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    • stebo02@sopuli.xyz ⁨1⁩ ⁨year⁩ ago

      math.stackexchange.com/…/how-to-prove-the-divisib…

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    • Ulvain@sh.itjust.works ⁨1⁩ ⁨year⁩ ago

      90°

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  • Iron_Lynx@lemmy.world ⁨1⁩ ⁨year⁩ ago

    And since both 3 and 17 are prime numbers, that makes 51 semiprime.

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    • KevonLooney@lemm.ee ⁨1⁩ ⁨year⁩ ago

      Which is not really rare under 100.

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    • Excrubulent@slrpnk.net ⁨1⁩ ⁨year⁩ ago

      Which is why it feels kind of prime, imho. I don’t know if other people get this, but I get a sense of which two-digit numbers are prime probably because of how often they show up in times tables and other maths operations.

      3*17 isn’t a common operation though and doesn’t show up in tables like that, so people probably aren’t generally familiar with it.

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    • WhiskyTangoFoxtrot@lemmy.world ⁨1⁩ ⁨year⁩ ago

      Do do do, do do do do.

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  • Sadbutdru@sopuli.xyz ⁨1⁩ ⁨year⁩ ago

    Does this also work the other way round, i.e. do all multiples of three have digits that sum to a multiple of 3? All the ones I’ve checked so far do, but is it proven?

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    • goddard_guryon@sopuli.xyz ⁨1⁩ ⁨year⁩ ago

      Indeed, an integer is divisible by 3 if and only if the sum of its digits is divisible by 3.

      For proof, take the polynomial representation of an integer n = a_0 * 10^k + a_1 * 10^{k-1} + … + a_k * 1. Note that 10 mod 3 = 1, which means that 10^i mod 3 = (10 mod 3)^i = 1. This makes all powers of 10 = 1 and you’re left with n = a_0 + a_1 + … + a_k. Thus, n is divisible by 3 iff a_0 + a_1 + … + a_k is. Also note that iff answers your question then; all multiples of 3 have to, by definition, have digits whose sum is a multiple of 3

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  • GladiusB@lemmy.world ⁨1⁩ ⁨year⁩ ago

    Username checks out

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  • fleabomber@lemm.ee ⁨1⁩ ⁨year⁩ ago

    Show off

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  • Black_Gulaman@lemmy.dbzer0.com ⁨1⁩ ⁨year⁩ ago

    Til thanks

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  • flambonkscious@sh.itjust.works ⁨1⁩ ⁨year⁩ ago

    Damn, logicbomb indeed!

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  • Fades@lemmy.world ⁨1⁩ ⁨year⁩ ago

    Oh, neat trick!

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• eager_eagle@lemmy.world ⁨1⁩ ⁨year⁩ ago

  51 = 317 317 = 17 + 17 + 17 17 + 17 + 17 = (10+7) + (10+7) + (10+7) (10+7) + (10+7) + (10+7) = 30 + 21 30 + 21 = 51

  yup, math checks out

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  • _dev_null@lemmy.zxcvn.xyz ⁨1⁩ ⁨year⁩ ago

    I think you skipped a step:

    1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1

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    • driving_crooner@lemmy.eco.br ⁨1⁩ ⁨year⁩ ago

      You miss a couple os steps too.

      First, lets define the axioms, we’re using Peano’s for this exercise.

      Axiom 1: 0 is a natural number.

      Jump to axiom 6, define the succession function s(n) where s(n) = 0 is false, and for brevity s(0) = 1, s(s(0)) = 2 and so on…

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  • BackOnMyBS@lemmy.world ⁨1⁩ ⁨year⁩ ago

    51 = 3*17

    3*17 = 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3

    3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 = (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1)

    (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) + (2+1) = 34 + 17

    34 + 17 = 51

    👌

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  • mbp@lemmy.sdf.org ⁨1⁩ ⁨year⁩ ago

    Image

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  • Poe@lemmy.world ⁨1⁩ ⁨year⁩ ago

    Math is mathing

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• theneverfox@pawb.social ⁨1⁩ ⁨year⁩ ago

  This is why I love the number 7. It’s the first real prime number. All the others are “first”…1?2?3?5? No, those aren’t prime numbers, they’re “first” in a long line of not-prime numbers.

  Then you get to 7. Is 27943 divisible by 7? If you take away 3 is it? If you add 4 is?

  I have no clue, give me 10 minutes or a calculator is the only answer

  That’s what a real prime number is.

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  • Karyoplasma@discuss.tchncs.de ⁨1⁩ ⁨year⁩ ago

    Take the last digit of the number, double it and subtract it from the rest. If that new number is divisible by 7, the original one is as well. For your example:

    2794 - 6 = 2788

    I know 2800 is divisible by seven, so 2788 is not. Thus 27943 is not divisible by 7.

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    • Tangent5280@lemmy.world ⁨1⁩ ⁨year⁩ ago

      Our plan to find the witch has worked, boys! Get her!

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    • match@pawb.social ⁨1⁩ ⁨year⁩ ago

      Quick check for divisibility: subtract 7 from it. If the new number is divisible by 7, then the original number is too

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    • AccountMaker@slrpnk.net ⁨1⁩ ⁨year⁩ ago

      But what about 14, 21 and 28?

      14 - 4*2 = 6, not divisible by 7

      21 - 1*2 = 19, not divisible by 7

      28 - 8*2 = 12, not divisible by 7

      Or did I misunderstand the algorithm?

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    • cactusupyourbutt@lemmy.world ⁨1⁩ ⁨year⁩ ago

      okay I understand that this works, but is there a mathematical proof for this?

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  • postmateDumbass@lemmy.world ⁨1⁩ ⁨year⁩ ago

    First non fibinochi prime

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  • saigot@lemmy.ca ⁨1⁩ ⁨year⁩ ago

    27943 - 71000 = 20943 20943 -73*1000 = -67

    -67 is not divisible by 7 therefore 27943 is not divisible by 7.

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    • theneverfox@pawb.social ⁨1⁩ ⁨year⁩ ago

      The other posters algorithm was better, but I was exaggerating - ultimately my point is you have to math it out

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  • SuddenlyBlowGreen@lemmy.world ⁨1⁩ ⁨year⁩ ago

    Was this comment made by the timecube guy?

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• Dagwood222@lemm.ee ⁨1⁩ ⁨year⁩ ago

  Any number where the individual digits add up to a number divisible by ‘3’ is divisible by 3.

  51 = 5+1 = 6, which is divisible by three.

  Try it, you’ll see it always works.

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  • letsgo@lemm.ee ⁨1⁩ ⁨year⁩ ago

    There are tricks like that for a lot of numbers. For 7, chop off the last digit, double it and add it to what’s left. Repeat as required. If the result is divisible by 7 then the original number was. eg: 356 -> 35+12=47 not db7. 357 =>35+14 both db7 so we don’t even need to do the add.

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    • Dagwood222@lemm.ee ⁨1⁩ ⁨year⁩ ago

      Clever.

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    • BruceDoh@sh.itjust.works ⁨1⁩ ⁨year⁩ ago

      14: 1 + 8 = 7. Not db7

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  • Zacryon@feddit.de ⁨1⁩ ⁨year⁩ ago

    One of the reasons why I love the number 3. There are other neat digit sum tricks, see for example for the numbers 1 to 30 here: en.m.wikipedia.org/wiki/Divisibility_rule

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    • Dagwood222@lemm.ee ⁨1⁩ ⁨year⁩ ago

      They didn’t teach stuff like this in school, which is silly. This is the kind of thing that a kid would eat up. It’s like they wanted to make sure people hated math.

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  • Laukidh@infosec.pub ⁨1⁩ ⁨year⁩ ago

    I knew that worked with 9. Hmm, does it work with 6?

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    • AEsheron@lemmy.world ⁨1⁩ ⁨year⁩ ago

      Technically it does work for 6, more literally. But it’s still just aiming for 3, not 6. That’s half of it, if the starting number is even and divisible by 3 then it is also divisible by 6.

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• Resol@lemmy.world ⁨1⁩ ⁨year⁩ ago

  Nobody told her that 100,000,001 is also divisible by 17

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  • OpenHammer6677@lemmy.world ⁨1⁩ ⁨year⁩ ago

    Holy crapballs

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  • moonburster@lemmy.world ⁨1⁩ ⁨year⁩ ago

    This one does more than the one OP showed

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    • Resol@lemmy.world ⁨1⁩ ⁨year⁩ ago

      Amirite

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• Ubettawerk@lemmy.blahaj.zone ⁨1⁩ ⁨year⁩ ago

  my palms are sweaty

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  • Ryan213@lemmy.world ⁨1⁩ ⁨year⁩ ago

    Mom’s spaghetti

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    • FiniteLooper@lemm.ee ⁨1⁩ ⁨year⁩ ago

      17

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• kicksystem@lemmy.world ⁨1⁩ ⁨year⁩ ago

  wait till she finds out that 0.99999… 9’s to infinity is the same as 1

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  • KeisukeTakatou@lemmy.world ⁨1⁩ ⁨year⁩ ago

    Lmao how about …99999 = -1?

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    • ledtasso@lemmy.world ⁨1⁩ ⁨year⁩ ago

      This one has always bothered me a bit because …999999 is the same as infinity, so it’s like you’re just doing math using infinity as a real quantity which we all know is invalid.

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• FlyingSquid@lemmy.world ⁨1⁩ ⁨year⁩ ago

  Math is hard, so I’m just going to assume that’s true and move on with my day.

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• forrgott@lemm.ee ⁨1⁩ ⁨year⁩ ago

  I love how every reply has like the opposite energy to the meme. I also find math to be generally awesome.

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  • radix@lemm.ee ⁨1⁩ ⁨year⁩ ago

    That’s Lemmy for you!

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• julianh@lemm.ee ⁨1⁩ ⁨year⁩ ago

  So is 100,000,001.

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• KSPAtlas@sopuli.xyz ⁨1⁩ ⁨year⁩ ago

  When you start playing modded minecraft you get really good at multiplying and dividing by 144

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  • cordlesslamp@lemmy.today ⁨1⁩ ⁨year⁩ ago

    Why 144? You mean a Minecraft stack of 64?

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    • Noodle07@lemmy.world ⁨1⁩ ⁨year⁩ ago

      Let me introduce you to the wonders of 144 millibuckets ingots

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• EmperorHenry@discuss.tchncs.de ⁨1⁩ ⁨year⁩ ago

  I don’t get it, why is this a big deal?

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  • stillwater@lemm.ee ⁨1⁩ ⁨year⁩ ago

    This is a shitpost

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    • EmperorHenry@discuss.tchncs.de ⁨1⁩ ⁨year⁩ ago

      I know…I don’t get what the joke is supposed to be though.

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• MystikIncarnate@lemmy.ca ⁨1⁩ ⁨year⁩ ago

  Technically, isn’t everything divisible by any number? You just get remainders and/or fractions in the result?

  I mean, I still didn’t want to know this, but…

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• AgentGrimstone@lemmy.world ⁨1⁩ ⁨year⁩ ago

  Why did she share this? Does she hate us? I don’t even know her.

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• BigBlackCockroach@lemmy.world ⁨1⁩ ⁨year⁩ ago

  Big, if true

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• Iron_Lynx@lemmy.world ⁨1⁩ ⁨year⁩ ago

  Upon closer inspection, yeah. 51 = 17 * 3

  = (10 + 7)*3

  = 103 + 73

  = 30 + 21 = 51

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• TimewornTraveler@lemm.ee ⁨1⁩ ⁨year⁩ ago

  weird how ppl are getting all excited over this. weirder all the random math facts on the comments.

  17 is a prime number

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• MrJameGumb@lemmy.world ⁨1⁩ ⁨year⁩ ago

  Oh no! 😰

  That made my back hurt.

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• Akasazh@feddit.nl ⁨1⁩ ⁨year⁩ ago

  This is the only one kind of math that professional darts players will dominate.

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• dbilitated@aussie.zone ⁨1⁩ ⁨year⁩ ago

  I actually really like this. 17 is three less than 20, 20x3 is 60, 3x3 is 9, 51 is 60-9. It just feels nice how it all fits together.

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• Cephirux@lemmings.world ⁨1⁩ ⁨year⁩ ago

  Curse you OP! Why did you post this?

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• fne8w2ah@lemmy.world ⁨1⁩ ⁨year⁩ ago

  17 * 3 baby!

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