Question

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

Answer 1

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Answer 2

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

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