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mitchelsewbaran

  • 3 years ago

A child is creating a pyramid with building blocks. The top three levels include 5 blocks, 12 blocks, and 19 blocks. How many blocks would be needed for a pyramid 25 levels tall? Use complete sentences to explain how a sum of an arithmetic series was applied.

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  1. tcarroll010
    • 3 years ago
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    [5 + (7*0)] + [5 + (7*1)] + [5 + (7*2)] + ... + [5 + (7*24)] = (25)(5) + 7(1 + 2 + 3 + ... + 24) = 125 + 7[(24)(25)]/2

  2. tcarroll010
    • 3 years ago
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    That's a lot of blocks for one child! :-)

  3. tcarroll010
    • 3 years ago
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    The key is that the sum of 1 + 2 + 3 + ... + (n-1) + n = n(n+1)/2

  4. tcarroll010
    • 3 years ago
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    Making sense? If not, ask any question you like.

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