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题库 / Prep2007E1-PS-37
For every integer k from 1 to 10, inclusive, the kth term of a certain sequence is given by ~$\left ( -1 \right )^{k+1}\left ( \frac{1}{2^{k}} \right )$~. If T is the sum of the first 10 terms in the sequence, then T is
greater than 2
between 1 and 2
between ~$\frac{1}{2}$~ and 1
between ~$\frac{1}{4}$~ and ~$\frac{1}{2}$~
less than ~$\frac{1}{4}$~
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a = 1/2 , q= -1/2, n = 10.
等比数列
S=( (1/2) * (1- (-1/2)^10) ) / (1-(-1/2)
S=(1023/1024 ) / 3
Since 1023/1024 is close to 1,
1/3 is between 1/2 and 1/4.
So the answer is D
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