Math

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A pile of blocks has 40 blocks in the bottom row 36 blocks in the second row 32 blocks in the third row and so on until there are only 4 blocks on the top row how many blocks? Are there in the 7th row? How many blocks are there in all

  • Math -

    (40-4)/4 = 9
    so, there are 9 rows after the 1st, making 10 in all.

    On the nth row, there are
    40-(n-1)*4 = 44-4n bricks

    The sum of n rows, Sn, with
    a = 40
    d = -4

    is

    Sn = n/2 (2*40 - (n-1)*4) = 2n(21-n)

    Now you can answer the questions.

  • Math -

    135

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