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Junior | Next Rank: 30 Posts
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GMAT practice test

by bronzie35 » Mon Apr 13, 2009 11:18 am
For every integer k from 1 to 10, inclusive, the kth term of a certain sequence is given by (-1) ^k+1 times (1/2^k). If T is the sum of the first 10 terms in the sequence, then T is which fo the following?

a. greater than 2
b. between 1 & 2
c. between 1/2 & 1
d. between 1/4 & 1/2
e. less than 1/4

Answer is d.

Thanks for your help!
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Master | Next Rank: 500 Posts
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good question

by artistocrat » Mon Apr 13, 2009 12:28 pm
Start with if k=1 (as k must be between 1 and 10), the first term of the sequence is (-1)^1+1 * (1/2^1) = 1/2. The next term if k=2 is (-1)^1+2 * (1/2^2) = -1/4.

Similarly, the next term(s) to k=10 will yield the pattern of terms: 1/8, -1/16, 1/32, -1/64 etc. So it is apparent that T (the sum of the first 10 terms in the sequence) is between 1/4 and 1/2, since 1/2-1/4+1/8-1/16...

Hope it helps.
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Junior | Next Rank: 30 Posts
Posts: 18
Joined: Fri Apr 03, 2009 3:33 pm

by bronzie35 » Mon Apr 13, 2009 1:00 pm
Ohhhhhh! Thank you. I didn't recognize the pattern.
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