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Sequential Subtraction

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bk_syed
 Post subject: Sequential Subtraction
 Post Posted: Tue Aug 10, 2010 12:19 pm 
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Course Students


Posts: 1
Sequence S is defined as Sn = 2Sn-1 – 2. If S1 = 3, then S10 – S9 =

This method of Plugging in the values is kind of long.

Can anyone explain me the alternate method given?

S2 – S1 = 1 or (20)
S3 – S2 = 2 or (21)
S4 – S3 = 4 or (22)
S5 – S4 = 8 or (23)

S10 – S9 = 28 = 256.


Thanks
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tim
 Post subject: Re: Sequential Subtraction
 Post Posted: Sat Sep 04, 2010 10:39 pm 
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ManhattanGMAT Staff


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Location: Southwest Airlines, seat 21C
yeah this is a great alternate approach - basically looking for a pattern in earlier cases. since the problem is looking for the difference between successive values, take a look at what happens when you take successive values at earlier stages of the process. then you can extrapolate to higher values of the sequence..

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Tim Sanders
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