1.Find the number of terms in each of the following geometric sequences.
729,243,81,...,1
2.If x+1,x+5,2x+4 are in geometric sequence,find the values of x.
Maths Question 非常急
2007-11-11 6:07 am
回答 (2)
2007-11-11 6:18 am
✔ 最佳答案
1. ratio= 243/729 = 1/3so
1 = 729(1/3)^n
1/729 = (1/3)^n
log (1/729) = n log (1/3)
n = 6
There are 6 terms in the GS.
2. (x+5)/(x+1) = (2x+4)/(x+5)
x^2 + 10x + 25 = 2x^2 + 6x + 4
x^2 - 4x - 21 = 0
(x+3)(x-7) = 0
x=-3 or x=7
2007-11-10 22:20:15 補充：
1. 1 = 729(1/3)^(n-1)1/729 = (1/3)^(n-1)log (1/729) = (n-1) log (1/3)n-1= 6n=7There are 7 terms in the GS.
收錄日期: 2021-04-13 21:34:47
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20071110000051KK04721