Math

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Calculate S(68) for the arithmetic sequence a(n) = (2/3)n + 1/6

87/2
273/6
139/3
4,726/3

  • Math -

    The standard formula for an A.P. is:
    a(n) = a(1) + (n-1)d

    => a(n) = (2/3)n + 1/6
    = 5/6 - 4/6 + (2/3)n
    = 5/6 - (n-1)(2/3)
    Hence, a(1) = 5/6
    d = 2/3

    Sum formula:

    S(n) = (n/2)[2a(1) + (n-1)d]
    Plug in the values to get the sum.

  • Math -

    I get the last one: 4,726/3

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