Struggling with the nine times table? I have a failsafe method | Adrian Chiles
www.theguardian.com
Apparently, nines are the hardest to grasp for primary school children. If only they’d learned how to cheat like me, says Adrian Chiles

Apparently, nines are the hardest to grasp for primary school children. If only they’d learned how to cheat like me, says Adrian Chiles

  • kescusay@lemmy.worldMEnglish
    1·
    1 day ago

    Fun pattern with 8s

    • 8x1 = 8. Add digits, you get 8.
    • 8x2 = 16. Add digits, you get 7.
    • 8x3 = 24. Add digits, you get 6.
    • 8x4 = 32. Add digits, you get 5.
    • 8x5 = 40. Add digits, you get 4.

    Now from there, you’d think the pattern breaks, but it’s actually just the first sequence in a larger pattern.

    • 8x6 = 48. Add digits (second sequence): 12. Add again: 3.
    • 8x7 = 56. Add digits: 11. Add again: 2.
    • 8x8 = 64. Add digits: 10. Add again: 1.

    Then it gets interesting:

    • 8x8 = 72. Add digits: 9. And now you finished the first sequence (from 8 to 0).
    • 8x10 = 80. Add digits: 8 (second sequence).
    • 8x11 = 88. Add digits (third sequence): 16. Add again (second sequence): 7
    • 8x12 = 96. Add digits: 15. Add again: 6.

    And it keeps going like that.

    • Aatube@lemmy.dbzer0.comOPEnglish
      2·
      23 hours ago

      that actually goes for every digit. when you add eight to a number n=10a+b=\overline{ab} (so a is in the tens digit and b is the ones digit), you can think of this as:

      1. add all of the eight to b
      2. if b is greater than 10 as a result, subtract 10 from b and add 1 to a

      what is 8-10? -2. so if you’re shifting the tens digit (a) up by 1, the ones digit is shifting down by 2; if you’re looking at the sum of digits, +1-2=-1.

      you can do similar things for every other digit added to a number. 8 is -2, 9 is -1, 7 is -3…