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GMAT Prep PS sequence

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GMAT Prep PS sequence [#permalink] New post 15 Mar 2009, 13:31
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0% (00:00) correct 0% (00:00) wrong based on 0 sessions
For every integer K from 1-10 inclusive kth term of a certain sequence is given by (-1)^K+1 .(1/2^K) . If T is the sum of the first 10 terms in a sequence, then T is

A. Greater than 2
B. Between 1 & 2
C. Between 1/2 & 1
D. Between 1/4 & 1/2
E. Less than 1/4

Can you please let me know how to solve it less than 2 minutes?
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Re: GMAT Prep PS sequence [#permalink] New post 16 Mar 2009, 05:31
Never mind, solved it:

5th term = (-1/2)^6*(1/2^5)=1/32
6th term = (-1/2)^7*(1/2^6)=-1/64
Average of the two= 1/32
Sum of 10 terms= 10*1/32=5/16----31% or between 1/4(25%) and 1/2(50%)

Is there any other quick approach?
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Re: GMAT Prep PS sequence [#permalink] New post 14 Apr 2009, 19:19
First find the r by plugging k as 1 and 2, in this case -1/2
since |r|<1, we can use the GP formula a/(1- r)
which in this case gives 1/3.

Hence answer D.
Re: GMAT Prep PS sequence   [#permalink] 14 Apr 2009, 19:19
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