請詳細步聚教我計以下二條 :
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急!F6 Arithmetic Sequences 01q5
2013-03-30 10:46 pm
回答 (1)
2013-03-30 11:41 pm
✔ 最佳答案
13(a) a = 7, d = 3S(18) = (18/2)[2 * 7 + 3(18 - 1)] = 9 * 65 = 585
(b) S(29) - S(18)
= (29/2)[2 * 7 + 3(29 - 1)] - 585
= 1421 - 585
= 836
14 a_4 + 3d = a_7
-3 + 3d = 9
d = 4
a_1 = -3 - 3d = -15
a_12 + ... + a_24
= S(24) - S(11)
= (24/2)[2(-15) + 3 * 23] - (11/2)[2(-15) + 3 * 10]
= 12 * 39 - 0
= 468
收錄日期: 2021-04-13 19:24:00
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20130330000051KK00148