GWD 13-6

uspresident

Q6:

The infinite sequence a₁, a₂,…,a_n,… is such that a₁ = 2, a₂ = -3, a₃= 5, a₄ = -1, and a_n = a_n-4 for n > 4. What is the sum of the first 97 terms of the sequence?

A. 72
B. 74
C.75
D.78
E.80
不是应该是75吗？ 为什么答案是74？
Thanks。

casualwark11

a1+a2+a3+a4=3

S97=S96+S97=s96+A1

S96=(96/4)*3=24*3=72

S97=72+A1=74

可能是搂主不细心算错了

uspresident

谢谢！！！