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PLEASE HELP
Find the sum of each series, if it exists.?
回答 (1)
2016-06-10 3:43 am
91 + 85 + 79 + … + (29) is an arithmetic sequence.
The first term, a₁ = 91
The common difference, d = 85 - 91 = 79 - 85 = -6
The last term, an = -29
an = a₁ + (n - 1)d
-29 = 91 + (n - 1) × (-6)
-6(n - 1) = -120
n - 1 = 20
n = 21
91 + 85 + 79 + … + (29)
= S(21)
= (1/2) × n × [a₁ + an]
= (1/2) × 21 × [91 + (-29)]
= 651
The first term, a₁ = 91
The common difference, d = 85 - 91 = 79 - 85 = -6
The last term, an = -29
an = a₁ + (n - 1)d
-29 = 91 + (n - 1) × (-6)
-6(n - 1) = -120
n - 1 = 20
n = 21
91 + 85 + 79 + … + (29)
= S(21)
= (1/2) × n × [a₁ + an]
= (1/2) × 21 × [91 + (-29)]
= 651
收錄日期: 2021-04-18 14:58:35
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https://hk.answers.yahoo.com/question/index?qid=20160609191033AAdaivt