f.6數(geometric sequences)20分

2011-10-24 5:51 am

我唔明點計兩條數...plz help me
1)Find the sum to infinity of the geometric sequence: -1,1/x ,-1/x^2 ,1/x^3 ,.......where x>1.

A)-1/x-1
B)-1/x+1
C)-x/x-1
D)-x/x+1
E)x/x+1

ANS:A

2)The nth term of a geometric sequence is -1/2^n, find the first term and the common ratio.

First term common raion
A. -1 1/2
B. -1/2 -1/2
C. -1/2 1/2
D. -1/2 1
E. 1 -1/2

ANS:C

我唔明點計,要詳細STEP
thx~~~~~~~

更新1:

回001: 第2條明晒..但係第1條,個model ans 係A,點解?

更新2:

我都計到係D,但係應該不會印錯吧-_-"

回答 (1)

myisland8132

2011-10-24 6:10 am

✔ 最佳答案

1 First term: -1Common ratio: -1/xSo, the sum to infinity of the geometric sequence= -1/(1 - (-1/x))= -X/(X + 1)ANS: D2 First term = -1/2^1 = -1/2Second term = -1/2^2 = -1/4So, common ration is 1/2ANS: C

2011-10-23 22:27:58 補充：
因為印錯了？﹗

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