Предмет: Алгебра, автор: vleninskiy95

найти сумму двадцати первых членов арифметической прогрессии , если а1 = 3, а2 = -3

Ответы

Автор ответа: Хуqожнuк

6

a2 = a1 + d
-3 = 3 + d
d = -6

a20 = a1 + 19d = 3 - 6*19 = -111

S20 = (a1 + a20)*20/2 = (3 - 111) * 10 = -108 * 10 = -1080

Ответ: -1080

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