求救 GWD 10-22

starryshen

GWD 10-22

In the sequence 1,2,4,8,16,32..........each term after the first is twice the previous term. what is the sum of the 16th, 17th and 18th terms in the sequence?

A. 2的18次方 B.3x(2的17次方) C.7x(2的16次方) D. 3x(2的16次方)

E.7x(2的15次方)

thanks for help............看见这种题都不知道怎么做，只知道和排列有关。

missouyang

第n个数是2的n-1次方

就是求2^15+2^16+2^17

按照等比数列求和公式就是a1(1-q^n)/(1-q)

具体到题目中，a1=2^15，q=2，n=3

代入就是7a1就是7*2*15

sunjueliang

2^15+2^16+2^17 提取公因数2^15,

得出2^15(1+2^1+2^2)=2^15*7