【求助一道PREP数学】

lovessie · 发表于 2010-8-12 18:03:37

For every integer k from 1 to 10, inclusive, the kth term of a certain sequence is given by(-1)的K+1次方*(1/2的k次方)
. If T is the sum of the first 10 terms in the sequence, then T is(A) greater than 2
(B) between 1 and 2
(C) between 1/2 and 1
(D) between 1/4 and 1/2
(E) between 1/4
想请教这道题怎么做。。。。公式打成这样了不知道大家能不能看懂。。。

feid1984 · 发表于 2010-8-12 18:12:52

a(k)=(-1)^(k+1) (1/2)^k

So a(1)= 1/2
a(2)= - (1/2)^2
a(3)= (1/2)^3
a(4)=- (1/2)^4
。。。。

a(10)=-(1/2)^10

相邻两项相加：
a(1)+a(2)=1/4
a(3)+a(4)=1/16
a(5)+a(6)=1/64
.........

肯定 >1/4 而<1/2

【求助一道PREP数学】

所属分类: GMAT考试

正在浏览此版块的会员 ()