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Joined 5 months ago
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Cake day: February 21st, 2026

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  • I know a couple of math tricks you can do with playing cards.

    • Give an audience member a pre-sorted deck of cards. Remove any card. Riffle shuffle the deck 3 times (4 if you’re bold). Return the removed card to a random position in the deck. Magician examines the deck and identifies the card.

    • Audience member selects any 5 cards and hands them the the magician’s beautiful assistant. Assistant examines these cards, and reveals 4 of them, one at a time, to the magician. Magician predicts the 5th card.



  • crwth@piefed.ziptoScience Memes@mander.xyz$1$
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    1 month ago

    I had a student tasked with summing a finite geometric sequence with |r|>1, let’s say 1+2+4+8+16. He had apparently forgotten the formula for that, but knew the formula for the infinite series a/(1-r). Good enough he thinks, and sums 1+2+4+… = -1, then subtracts off the excess terms 32+64+128+… = -32, and gets the correct answer of 31.