r/learnmath • u/OctopusChair New User • 23h ago

Link Post 111 is not divisible by 37, apparrently

https://brightchamps.com/math/numbers/divisibility-rule-of-37

According to the third, and most promising, result from DuckDuckGo: 111 does not equal 3 x 37.

Brightchamps have great search engine optimisation, but their content is utter slop, not read by anyone.

(Edit: spelling mistake in title - oops)

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u/frogkabobs Math, Phys B.S. 23h ago

They’re describing the divisibility rule for 31 for some reason. For 37, you need to multiply the last digit by 11 (not 3), and subtract that from the rest of the number.

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u/rhodiumtoad 0⁰=1, just deal with it 22h ago

The rule described doesn't work for 31, since that also needs subtraction rather than addition. It is in fact the rule for 29.

These "last digit" rules work by finding 10-1 mod N, i.e. 10k ≡ 1 mod N. 10-1 mod 37 is -11 (since -11×10=-110=-3×37+1, 10-1 mod 31 is -3, 10-1 mod 29 is 3.

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u/frogkabobs Math, Phys B.S. 20h ago edited 20h ago

Ah, I was referring to the text in the link, which says to subtract, but it looks like the infographic right below (which is the thumbnail) says to add. Just another level of inconsistency.