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[-] beefbot@lemmy.blahaj.zone 27 points 5 days ago

Some superstitious nerdy teenagers convinced small child me that 91 was a universal anomaly bc it’s 13 (bad luck) * 7 (good luck), so my weird brain never saw 91 as prime , & I can’t be the only person this happened to

[-] mvirts@lemmy.world 18 points 5 days ago
[-] TunaCowboy@lemmy.world 29 points 5 days ago

Naw, 5 + 1 = 6, 6 is divisible by 3, so 51 is divisible by 3.

[-] xpinchx@lemmy.world 4 points 5 days ago

I shared this at work recently and nobody cared. 3 6 and 9 are all kinda special.

[-] ShareMySims@sh.itjust.works 19 points 5 days ago* (last edited 5 days ago)

Only if you weren't taught (or were, but too long ago lol) the divisible by 3 rule - if the sum of the digits is divisible by 3, so is the whole number. 5+1=6 therefore 51 is divisible by 3! Bonus division rule - if it was also an even number and therefore divisible by 2, you could deduce that the whole number was divisible by 6.

[-] Xtallll@lemmy.blahaj.zone 22 points 5 days ago
[-] Cadeillac@lemmy.world 8 points 5 days ago

You bastard

!unexpectedfactorial@sopuli.xyz

[-] Nougat@fedia.io 4 points 5 days ago

Actually, 51 is not divisible by 3!

It is divisible by 3

[-] ShareMySims@sh.itjust.works 1 points 5 days ago
[-] smeg@feddit.uk 3 points 5 days ago

Surely it's 1? Looks prime, and some people will even argue that it is!

[-] mvirts@lemmy.world 3 points 5 days ago

Its only factors are 1 and itself after all

[-] demonmittenhands@lemmy.world 3 points 5 days ago

Agreed. 51 looks prime AF.

[-] erie09@lemmy.world 6 points 5 days ago

I always felt that 21 should be, but nooo. Stupid 7

[-] threelonmusketeers@sh.itjust.works 4 points 5 days ago* (last edited 5 days ago)

21 should be

2 + 1 = 3, therefore 21 is divisible by 3 and is not prime. QED :)

[-] BigDanishGuy@sh.itjust.works 2 points 5 days ago

9+1 = 10, therefore 91 is divisible by 10 and is not a prime ... QED, I guess

[-] humorlessrepost@lemmy.world 1 points 4 days ago

In the right base, that might work.

[-] lunarul@lemmy.world 2 points 5 days ago

It's funny, but actually a good proof

this post was submitted on 22 Sep 2024
103 points (97.2% liked)

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