求助GWD5-8

dhhgh11

Q8:

The infinite sequence a1, a2,…, an,… is such that a1 = 2, a2 = -3, a3 = 5, a4 = -1, and an = an-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

请教思路,谢谢

himba

我也觉得选B，因为每四个数的数值是一样的，那前97个数里面就有24个这样的，97/4=24余1。那就是24*（2-3+5-1）+2=74。

不知道对不对？