Помогите с 11 номером!!!!!

Question

Помогите с 11 номером!!!!!

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Answer 1

a1 = -8.2

an+1 = an - 2.3

d = -2.3

a15 = a1 + d · 14 = -8.2 - 2.3 · 14 = -40.4

S15 = 0.5 · (a1 + a15) · 15 = 0.5 · (-8.2 - 40.4) · 15 = -364.5

Ответ: -364,5