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Thread: Sum of numbers on a line in a spiral

  1. #1
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    Sum of numbers on a line in a spiral

    Sum of numbers on a line in a spiral-csmc17a5.pngI think there is a simpler way to do this but all I thought of was finding the equation of the numbers on the line. Can someone help me how to do this more simply?
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  2. #2
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    Re: Sum of numbers on a line in a spiral

    points on the line $y=-x$ are given by

    $n(n+1)+1,~0 \leq n$

    a little toying shows that $(31)(32)+1=993$ is the largest one of these numbers 1000 or less.

    so we have

    $N = \sum \limits_{n=0}^{31} n(n+1)+1= 10416 + 496 + 31 = 10943$
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  3. #3
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    Re: Sum of numbers on a line in a spiral

    I keep getting a different answer, off by 1, but I think I know why:

    $\displaystyle \sum _{i=0} ^{31} 1= 32$, not 31, so final answer is 10944. What clued me in is that each of the diagonals is an odd number, so having an even number of entries on the diagonal would mean the sum must be even.
    Last edited by ChipB; Oct 7th 2018 at 04:05 PM.
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