這題想了很久也想不到,書中也沒例題所以來發問,希望有人可幫忙~
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Maths等比數列一問
2011-09-28 10:27 pm
回答 (1)
2011-09-28 11:31 pm
✔ 最佳答案
a)設T(n)=aR^(n-1) a是首項，R是公比
a+aR=4 -----(1)
a+aR+aR^2 = 14/3 -----(2)
a+aR+aR^2+aR^3 = 40/9 -----(3)
put (2) into (3), we have
aR^3 = 40/9 - 14/3 =-2/9 ----- (4)
put (1) into (2), we have
aR^2 = 14/3 -4 =2/3 ----(5)
(4)/(5), we have
R = (-2/9) / (2/3)
=-1/3
put it into (5), we have
a(-1/3)^2 = 2/3
a=6
b) Sum = a/(1-R) = 6/(1+1/3) = 9/2
希望幫到你！
收錄日期: 2021-04-11 18:48:22
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20110928000051KK00287