Question

Найдите сумму n первых челнов арифметической прогрессии: 8;3;-2;..., если n=9

Answer 1

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Answer 2

$a _{1} = 8 \\ d = - 5 \\ n = 9$

$S_{9} = \frac{2a_{1} + 8d }{2} \times 9 \\ S_{9} = \frac{2 \times 8 + 8 \times ( - 5)}{2} \times 9 = \\ = \frac{16 - 40}{2} \times 9 = -12 \times 9 =- 108$