Question

17
Σ (15 – 9n)
η = 4

Answer 1

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

Answer 2

-1113

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