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15 February, 10:34

17

Σ (15 - 9n)

η = 4

+5
Answers (2)
  1. 15 February, 10:43
    0
    -1113

    Step-by-step explanation:

    You apparently want the sum of the 14-term arithmetic sequence ...

    -21, - 30, - 39, ..., - 138

    The average term is ...

    (-21 - 138) / 2 = - 79.5

    so the sum of the 14 terms is ...

    (14) (-79.5) = - 1113
  2. 15 February, 10:54
    0
    17

    Σ (15 - 9n) = - 1113

    η = 4

    nth term

    aₙ = 15 - 9n

    a₄ = 15-9*4 = 15-36 = - 21

    a₁₇ = 15 - 9 (17) = - 138

    Sum;

    S = (a₁ + aₙ) * n/2

    n = 14

    a₁ = a₄ = - 21

    aₙ = a₁₇ = - 138

    S = (-21 - 138) * 14/2

    S = - 1113
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