A Multiplication Based Logic Puzzle

385 is the sum of the squares of the first ten counting numbers. Let me demonstrate what that means. All of the following are square pyramidal numbers:

  • 1² = 1
  • 1² + 2² = 5
  • 1² + 2² + 3² = 14
  • 1² + 2² + 3² + 4² = 30
  • 1² + 2² + 3² + 4² + 5² = 55
  • 1² + 2² + 3² + 4² + 5² + 6² = 91
  • 1² + 2² + 3² + 4² + 5² + 6² + 7² = 140
  • 1² + 2² + 3² + 4² + 5² + 6² + 7² + 8² = 204
  • 1² + 2² + 3² + 4² + 5² + 6² + 7² + 8² + 9² = 285
  • 1² + 2² + 3² + 4² + 5² + 6² + 7² + 8² + 9² + 10² = 385

We could have found the total, 385, much more quickly by putting the number ten in for “n” in this formula

Which means the sum of the first ten square counting numbers = (10 x 11 x 21)/6 = 5 x 11 x 7 = 385. (Notice its prime factorization!)

Here’s a video explanation:

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Comments on: "385 Is a Square Pyramidal Number" (3)

  1. This is so cool.

    Liked by 1 person

  2. ivasallay said:

    I thought so, too!

    Like

  3. […] is the 12th square pyramidal number and can be calculated using 12(12 +1)(2⋅12 + […]

    Like

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