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急!F6 Arithmetic Sequences 01q4
2013-03-30 10:44 pm
回答 (1)
2013-03-30 11:46 pm
✔ 最佳答案
11 S(37) = 37 * 38 = 1406 and S(36) = 1332a_37 = S(37) - S(36) = 74
12 a_3 + a_4 = -1, S(15) = 195
So, a + 2d + a + 3d = -1 or 2a + 5d = -1...(1)
(15/2)(2a + 14d) = 195 or a + 7d = 13...(2)
Solveit, a = -8, d = 3
S(26) = (26/2)[2(-8) + 3(26 - 1)]
= 13 * 49
= 637
收錄日期: 2021-04-13 19:24:18
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20130330000051KK00147