Task/28220675
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Найти восьмой член и сумму первых восьми чисел геометрической прогрессии, если b₁= - 18, q=1/3.
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b₈ =b₁*q⁷ = - 18*(1/3)⁷ = -2*3² /3⁷ = -2/3⁵ = - 2/243 .
S₈ =b₁(1 - q⁸) / (1-q) * * * = b₁(q⁸ - 1) / (q -1) * * *
S₈ =(b₁ - b₁q⁷*q ) / (1- q) =(-18 +(2/243) *(1/3) ) / (1-1/3) =
(-18 +(2/243) *(1/3) ) * (3/2) = - 27 +1/ 243 = [ - 26 ] 242/243 .