the geometric sequence problem, thanks?
2018-11-22 12:47 pm
consider the geometric sequence 1,-3,9,....If the sum of the first n terms is 547, find the value of n
回答 (2)
2018-11-22 7:16 pm
✔ 最佳答案
consider the geometric sequence 1，－3，9，....If the sum of the first n terms is 547, find the value of nSol
a(n)=(－3)^(n－1)
S(n)=1+(－3)+9+(－27)+…+(－3)^(n-1)
－3S(n)=(－3)+9+(－27)+…+(－3)^(n-1)+(－3)^n
－3S(n)－S(n)=(－3)^n－1
－4S(n)=－4*547=(－3)^n－1
(－3)^n=－2188+1=－2187
n=7
2018-11-23 1:16 pm
using theory
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https://hk.answers.yahoo.com/question/index?qid=20181122044734AAomopD