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What is the quickest way to find the value of a certain term

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What is the quickest way to find the value of a certain term [#permalink]

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New post 30 Sep 2004, 12:32
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

What is the quickest way to find the value of a certain term in a progression?

For example:

20, 40, 80, 160, 320...

What's the quickest way to find out what the 20th term in that sequence is?

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New post 30 Sep 2004, 12:50
I cant think of any quicker way other than to use GP (Geometric progression).

If you look at the sequence, you see the numbers doubling OR we can say that the numbers have a common ratio - in this case '2'. The formula is
nth term = a*(r^n-1). a= first term. r= common ratio and n=nth term

The 20th term will be = 20x2^(20-1) = 20x2^19 = 10485760

Last edited by venksune on 30 Sep 2004, 12:51, edited 1 time in total.

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New post 30 Sep 2004, 12:50
a(r^ (n-1) )
a is the first term , r the common factor, n the number of term ins equence.
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New post 30 Sep 2004, 13:03
Thanks, I had a question related to this topic on my GMAT & had no clue how to do it in 2 minutes & had to guess.

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