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Intern
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Ps-Gmat prep [#permalink] New post 03 Nov 2006, 20:37
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A
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C
D
E

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Q2. In the arithmetic sequence T1, T2, T3, …….Tn,…..,T1=23 and Tn=Tn-1 -3 for eah n>1. when tn = -4 what is value of n?

a.-1
b.7
c.10
d.14
e.20




Answer. Choice c

wud appreciate it if someone can explain how.
Senior Manager
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 [#permalink] New post 03 Nov 2006, 20:49
I think its a good idea to understand how to derive a sequence, but in this case there is less room for error and stress if you just write the numbers down.

If T1=23, Tn-1 - 3 means -3 for each successive term, so:

T1 = 23
T2=20
17
14
11
8
5
2
-1
T10=-4

so 10.
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 [#permalink] New post 03 Nov 2006, 21:20
Thanks guys. was trying to work out the problem by sequence. which as u can see i wasn't able to set up properly. :oops: but its nice to know that could have solved it without working out sequence.
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 [#permalink] New post 04 Nov 2006, 02:56
***** Reposted followring some troubles ******

One way is to identify a pattern. :)

Since Tn=Tn-1 -3, we have

o T2=T1 -3
o T3=T2 -3 = T1 - 3*2
o T4=T3 -3 = T1 - 3*3

Thus, Tn=T1 -3*(n-1)

Tn=T1 -3*(n-1)
<=> -4 = 23 - 3*(n-1)
<=> n-1 = 27/3
<=> n = 10
  [#permalink] 04 Nov 2006, 02:56
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