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Infinite Seq

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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Infinite Seq

by kstv » Wed May 05, 2010 5:29 pm
1. The infinite sequence a1, a2,..., an,... is such that a1 = 2, a2 = -3, a3 = 5, a4 = -1, and an = an-4 for n > 4. What is the sum of the first 97 terms of the sequence?
A. 72
B. 74
C. 75
D. 78
E. 8

This is one of the 5 Q posted by Bibha in a single post. This Q deserves better to stand out.
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by liferocks » Wed May 05, 2010 6:03 pm
97=4*24 + 1
since an = an-4 for n>4 the sum of the series

will be 24*(a1+a2+a3+a4)+a1=24(2+-3+5-1)+2=72+2=74

Ans option B

Thanks for posting this separately..I would have missed this otherwise!
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