maths
2011-07-30 6:06 am
The 1st termand the 2nd term of an arithmetic sequence are 5 and 8 respectively. If the sumof the first n terms of the sequence is 3925, find n
回答 (1)
2011-07-30 6:50 am
✔ 最佳答案
Let the 1st term be a and the common difference be d.The nth term is: T(n)=a+(n-1)d
The sum of the first n terms is: S(n)=n[2a+(n-1)d]/2
T(1)=5
T(2)=5+(2-1)d=8
5+d=8
d=3
S(n)=n[2X5+3(n-1)]/2=3925
n[10+(3n-3)]=7850
n(3n+7)=7850
3n^2+7n-7850=0
(n-50)(3n+157) =0
n-50=0 or 3n+157=0
n=50 or n=-157/3 (rejected, n is a positive integer)
n=50
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https://hk.answers.yahoo.com/question/index?qid=20110729000051KK01306