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A comment on Why are Soviet math textbooks so hardcore in comparison to US textbooks? (2017). Parent comment

Re:Why are Soviet math textbooks so hardcore in comparison t... by DmitryOlshansky on Wednesday April 22, 2020 @04:17AM (#22943347)
Yes, a simple consequence of e.g. 3-digit number being:

100x + 10y + z modulo 3

Removing 9y and 99x gives equivalence: x + y + z modulo 3

Now what’s left is induction with proper base (1-digit).

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Re:Why are Soviet math textbooks so hardcore in comparison t... by red_admiral on Wednesday April 22, 2020 @04:46AM (#22943493)
As dan-robertson mentioned in another branch, you don't even need induction - you can sidestep it by writing X = sum_i x_i 10^i and noticing that all the 10^i are 1 modulo 9 (and therefore modulo 3).
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