Respuesta :

sqdancefan

Answer:

  -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

alexrocal

                                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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