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The New Coffee Room

  1. TNCR
  2. General Discussion
  3. Puzzle time - prime test

Puzzle time - prime test

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

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

    Only non-witches get due process.

    • Cotton Mather, Salem Massachusetts, 1692
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    • jon-nycJ Online
      jon-nycJ Online
      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.

      Only non-witches get due process.

      • Cotton Mather, Salem Massachusetts, 1692
      1 Reply Last reply
      • jon-nycJ Online
        jon-nycJ Online
        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.

        Only non-witches get due process.

        • Cotton Mather, Salem Massachusetts, 1692
        1 Reply Last reply
        • KlausK Online
          KlausK Online
          Klaus
          wrote on last edited by Klaus
          #4

          :::

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

          It's a square number. End of story.

          :::

          1 Reply Last reply
          • jon-nycJ Online
            jon-nycJ Online
            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

            Only non-witches get due process.

            • Cotton Mather, Salem Massachusetts, 1692
            1 Reply Last reply
            • jon-nycJ Online
              jon-nycJ Online
              jon-nyc
              wrote on last edited by
              #6

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

              Only non-witches get due process.

              • Cotton Mather, Salem Massachusetts, 1692
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              • KlausK Online
                KlausK Online
                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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