Prep1 又一数学 请教

Prep 1
For every integer K from 1 to 10, inclusive, the kth term of a certain sequence is give by [(-1)^(k+1)]*[1/(2^K)]. If T is sum of the first 10 terms in the sequence, the T is:

A >2
B (1,2)
C(1/2,1)
D(1/4,1/2)
E <1/4

请教思路。多谢了。
似乎以前见过此题，但是现在搜不到了。所以再贴。

等比数列

q为-1/2

观察发现是等比数列， 求和就可以了。

K取证无穷，，sum=1/3

K=10， sum略小于1/3

多谢各位热心帮助。