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The New Coffee Room

  1. TNCR
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  3. Puzzle time - prime test

Puzzle time - prime test

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  • jon-nycJ Offline
    jon-nycJ Offline
    jon-nyc
    wrote on last edited by jon-nyc
    #1

    Does 4^9 + 6^10 + 3^20 happen to be a prime number?

    "You never know what worse luck your bad luck has saved you from."
    -Cormac McCarthy

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    • jon-nycJ Offline
      jon-nycJ Offline
      jon-nyc
      wrote on last edited by
      #2

      I cheated and calculated the sum and checked it against known primes so I have the answer. But I’m sure there’s a clever answer.

      "You never know what worse luck your bad luck has saved you from."
      -Cormac McCarthy

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      • jon-nycJ Offline
        jon-nycJ Offline
        jon-nyc
        wrote on last edited by
        #3

        I have the clever solution. They could have made this problem with larger numbers so the brute force method was not possible.

        "You never know what worse luck your bad luck has saved you from."
        -Cormac McCarthy

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        • KlausK Offline
          KlausK Offline
          Klaus
          wrote on last edited by Klaus
          #4

          :::

          (a-b)^2 = a^2-2ab+b^2.

          It's a square number. End of story.

          :::

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          • jon-nycJ Offline
            jon-nycJ Offline
            jon-nyc
            wrote on last edited by jon-nyc
            #5

            Yep, and to my point that they could have made it not solvable through brute force, it works for any 3^n and 2^(n-1).

            So ask about 4^999 + 6^1000 + 3^2000

            "You never know what worse luck your bad luck has saved you from."
            -Cormac McCarthy

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            • jon-nycJ Offline
              jon-nycJ Offline
              jon-nyc
              wrote on last edited by
              #6

              2 weeks in a row they gave easy problems. Kinda disappointing.

              "You never know what worse luck your bad luck has saved you from."
              -Cormac McCarthy

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              • KlausK Offline
                KlausK Offline
                Klaus
                wrote on last edited by
                #7

                But, to be honest, it was easier to derive the "clever" solution from the "brute force" solution than arriving directly at the "clever" soluton.

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