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GMAT PREP (Arithmetic Sequence)

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GMAT PREP (Arithmetic Sequence)

by relaxin99 » Mon Feb 16, 2009 4:36 pm
In the arithmetic sequence t1, t2, t3......tn....., t1 = 23... tn = tn-1 - 3 for each n>1. What is the value of n when tn= -4

A) -1
B) 7
C) 10
D) 14
E) 20


Can someone please explain their answer as well, the correct answer is down below
























The answer is C
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by billzhao » Mon Feb 16, 2009 4:42 pm
We know that tn=(tn-1)-3, so the sequence is with arithmetic progression with tolerance: d = -3.

We have tn=t1+(n-1)*d, where t1=23, tn=-4 and d=-3, so we can solve the equation and find n.

-4=23-3*(n-1) =>n=10
Yiliang
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by tini » Mon Feb 16, 2009 5:01 pm
tn can be expressed as tn=t1-(n-1)3

-4=23-(n-1)3
n-1=27/3=9
n=10
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