• Count Regal Inkwell@pawb.social
    link
    fedilink
    arrow-up
    11
    ·
    3 hours ago

    “This is called ‘maths’. Or ‘math’ if you’re American, as they’re only allowed to have one. Due to… I don’t know budget cuts or something”. — ASHEN, Stuart

  • Nougat@fedia.io
    link
    fedilink
    arrow-up
    42
    ·
    1 day ago

    51 --> 5+1 = 6, 6 is divisible by 3. This means that 51 is divisible by 3.

    60 is divisible by 3, 60/3 = 20.

    51 is 9 less than 60. 9 is divisible by 3. 9/3 = 3.

    20 - 3 = 17.

    • denial@feddit.org
      link
      fedilink
      English
      arrow-up
      17
      ·
      edit-2
      1 day ago

      Only way I managed it to make sense is:

      17 is 10 and 7

      10 * 3 = 30

      7 * 3 = 21

      30 + 21 = 51

      Phuu air. I can breath again. Don’t do this to me.

      • rebelsimile@sh.itjust.works
        link
        fedilink
        arrow-up
        2
        ·
        24 hours ago

        This is how I see it, 30 + 21, doesn’t come up that often obviously, but also we don’t have to love every composite number. In fact, we hates most of them. Add 51 to the pile.

      • testfactor@lemmy.world
        link
        fedilink
        arrow-up
        37
        ·
        1 day ago

        Yep. If the sun of the numbers is divisible by 3, the number is divisible by three.

        Works great for 6 too, as if it’s divisible by 3 and even, the number is divisible by 6.

        And 9 is the same thing, but the sum has to be divisible by 9 (e.g. 12384 is divisible by 9 because the sum of the digits is 18, which is divisible by 9)

        There’s also good rules for 4 and 8 as well. If the last 2 digits are divisible by 4, the whole number is (e.g. 127924 is divisible by 4 because 24 is) and if the last 3 numbers are divisible by 8, the whole number is (e.g. 12709832 is divisible by 8 because 832 is.)

        • brian@lemmy.ca
          link
          fedilink
          arrow-up
          8
          ·
          1 day ago

          You just casually dropping in that 832 is divisible by 8 makes me feel as if there’s a small gap in our abilities to do mental math

          • sudo@lemmy.today
            link
            fedilink
            arrow-up
            9
            ·
            1 day ago

            832 is 800 + 32

            800 is obviously divisible by 8, so it can also be negated like the first few digits. 32 is also divisible by 8.

      • Seeker of Carcosa@feddit.uk
        link
        fedilink
        English
        arrow-up
        10
        ·
        1 day ago

        Divisibility by 3 rule is real. If the sum of the digits of a number is divisible by 3, then the number itself is also divisible by 3. Same goes with 9. There’s an 11 rule, but it’s a bit convoluted.

  • ArbitraryValue@sh.itjust.works
    link
    fedilink
    English
    arrow-up
    31
    ·
    1 day ago

    What blew my mind is this. What is the sum of the infinite series

    1, -1, 1, -1, ...

    One answer is to look at it like this:

    (1 - 1) + (1 - 1) + ... = 0

    Another answer is to look at it like this:

    1 + (-1 + 1) + (-1 + 1) + ... = 1

    But then it gets weirder. What if you add two of the series together like so:

    1 + -1 + 1 + -1 + ...

    ____ 1 + -1 + 1 + ...

    (Please ignore the underscores. They’re just there because otherwise Lemmy messes up the whitespace.)

    All the terms cancel out except that first 1 again. But this time it’s the sum of two of these series, which means that the sum of one series is 0.5 and somehow not an integer.

    The correct answer is that you’re not allowed to add up infinite series like this so that’s why you get contradictory results if you try.

    • yetAnotherUser@discuss.tchncs.de
      link
      fedilink
      arrow-up
      9
      ·
      1 day ago

      You are actually allowed to add up infinite series like this.

      Only that the infinite series have to be convergent, or else you get little of value. The series in your example oscillates forever (and the oscillation distance remains constant), therefore it diverges.

      Take the infinite series 1 + 0 + 0 + 0 + ... and add it like you did:

      1 + 0 + 0 + 0 + 0 + ...
      ___ 1 + 0 + 0 + 0 + ...

      And you just get 1 + 1 + 0 + 0 + 0 + ... which is just 2 * (1 + 0 + 0 + 0 + ...)

  • Zugyuk@lemmy.world
    link
    fedilink
    arrow-up
    4
    ·
    1 day ago

    That’s why you always go for multiples of 6 plus or minus 1 that are not multiples of 5 or 11.