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Patterns / Sequences

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Master | Next Rank: 500 Posts
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Patterns / Sequences

by doclkk » Tue Jul 28, 2009 4:47 pm
A certain computer program generates a sequence of numbers a1, a2, … , a n such that a1 = a2 = 1 and ak = ak-1 + 2ak-2 for all integers k such that 3 ≤ k ≤ n. If n > 6, then a7 = ?

A. 32
B. 43
C. 64
D. 100
E. 128

OA: B

This seems harder than the GMAT prep problem.
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by prindaroy » Tue Jul 28, 2009 6:44 pm
simply plug in numbers;

a4 = 3 + 2 = 5
a5 = 5 + 6 = 11

so and so forth

a7 = 43
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by PussInBoots » Tue Jul 28, 2009 7:27 pm
Plugging in the numbers should take you no more than 30-40 seconds. If anything, this problem is a gift.
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