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sequence

by orel » Mon Nov 24, 2008 10:34 pm
In the arithmetic sequence t1,t2,t3,....tn,...t1=23 and 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

please, explain!
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Re: sequence

by logitech » Mon Nov 24, 2008 11:30 pm
Feruza Matyakubova wrote:In the arithmetic sequence t1,t2,t3,....tn,...t1=23 and 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

please, explain!
I just solved this in my GMAT practice test :D

t2 - t1 = -3
t3 - t2 = -3
tn - t(n-1)= -3

+
--------------------

tn - t1 = -3(n-1)
-4-(-23)=-3(n-1)

-27=-3(n-1)

n=10
LGTCH
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by orel » Tue Nov 25, 2008 12:00 am
thanks,
but i still don't understand n-1 in the second part of your explanation
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by logitech » Tue Nov 25, 2008 12:19 am
t2 - t1 = -3
t3 - t2 = -3
+
--------------------

t3-t1 = 2x(-3)

t2 - t1 = -3
t3 - t2 = -3
t4 - t3 = -3

+
---------------

t4-t1 = 3x(-3)

Hope it is clear now.
LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"
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