(a-1)+(a^2-2)+....+(a^n-n)
求和..
等比數列求和ar....
2010-03-25 6:17 am
回答 (2)
2010-03-25 8:31 am
✔ 最佳答案
(a-1)+(a^2-2)+....+(a^n-n)= (a + a^2 + .... + a^n) - (1 + 2 + .... + n)
= a(1 - a^n)/(1 - a) - n(n+1)/2
= [ 2a(1 - a^n) - n(n+1)(1 - a) ] / [2(1 - a)]
2010-03-25 7:57 am
利用a(1-R^n)/(1-R)。
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原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20100324000051KK01708