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Struggling with the nine times table? I have a failsafe method (www.theguardian.com)
submitted 3 months ago by to c/nottheonion@lemmy.world
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Apparently, nines are the hardest to grasp for primary school children. If only they’d learned how to cheat like me, says Adrian Chiles

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[–] 1 point 3 months ago (1 child)

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.

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  • [–] [S] 2 points 3 months 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...

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