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GPREP DS

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by aj5105 » Thu Jan 15, 2009 11:51 pm
The terms of a sequence are t1, t2, t3......tn what is the value of n?

1. The sum of n terms is 3124

2. The average (arithmatic mean) of n terms is 4

Statement 1 -- is it fine to use the formula n(n+1)/2 = 3124 ?
I can simplify the quadratic equation and get the value of n.In that case A alone is sufficient.


Comments please.
Last edited by aj5105 on Fri Jan 16, 2009 12:38 am, edited 1 time in total.
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Source: — Data Sufficiency |
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Re: GPREP DS

by logitech » Thu Jan 15, 2009 11:56 pm
aj5105 wrote:f the terms of a sequence are t1, t2, t3......tn what is the value of n?

1. The sum of n terms is 3124

2. The average (arithmatic mean) of n terms is 4

Statement 1 -- is it fine to use the formula n(n+1)/2 = 3124 ?
I can simplify the quadratic equation and get the value of n.In that case A alone is sufficient.


Comments please.
what if

t1=4 and t2=5

The sum is = 9

Use nx(n+1)/2 for series as below:

1,2,3,4,5,6,7,.............., n

But you can also use the same formula to calculate:

6,7,8

=SUM(1-8) - SUM(1-5)

= 8x9/2 - 5x6/2
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by aj5105 » Fri Jan 16, 2009 1:24 am
got mislead by t1, t2, t3......tn
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by thechamp » Sun Jan 18, 2009 1:17 pm
What's the OA? Is it E?

The given sequence t1,t2,..tn does not say what kind of sequence it is (Arithmatic or Geometric). So, the answer should be E, right?
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by thechamp » Sun Jan 18, 2009 1:18 pm
What's the OA? Is it E?

The given sequence t1,t2,..tn does not say what kind of sequence it is (Arithmatic or Geometric). So, the answer should be E, right?
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by Ian Stewart » Sun Jan 18, 2009 1:40 pm
thechamp wrote:What's the OA? Is it E?

The given sequence t1,t2,..tn does not say what kind of sequence it is (Arithmatic or Geometric). So, the answer should be E, right?
There are lots of different possible types of sequences - they aren't all arithmetic or geometric. For example, this sequence:

1, 11, 111, 1111, 11111, ...

is neither arithmetic nor geometric. My point is, don't assume a sequence question has anything to do with arithmetic or geometric progression unless the question tells you about the details of the sequence. The above question really isn't about sequences at all; it's about a set of n values. Their average is 4 and their sum is 3124, so we have:

avg = sum/n
n = sum/avg
n = 3124/4

The answer is C.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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