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The infinite sequence a1, a2,...,an is defined such that an

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The infinite sequence a1, a2,...,an is defined such that an [#permalink] New post 23 Jul 2008, 07:20
The infinite sequence a1, a2,...,an is defined such that an = (n+2) / n for all n ≥ 1. What is the product of the first 10 terms of the sequence?

(A) 45
(B) 66
(C) 90
(D) 121
(E) 132
Quick way to solve this.
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Re: The Infinite sequence [#permalink] New post 24 Jul 2008, 06:19
Capthan wrote:
The infinite sequence a1, a2,...,an is defined such that an = (n+2) / n for all n ≥ 1. What is the product of the first 10 terms of the sequence?

(A) 45
(B) 66
(C) 90
(D) 121
(E) 132
Quick way to solve this.


The first ten terms are 3/1, 4/2, 5/3, ..., 11/9, 12/10

The product of these terms is (12*11*10*...*4*3)/10! = 12*11/2 = 66
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Re: The Infinite sequence [#permalink] New post 24 Jul 2008, 08:38
IanStewart wrote:
Capthan wrote:
The infinite sequence a1, a2,...,an is defined such that an = (n+2) / n for all n ≥ 1. What is the product of the first 10 terms of the sequence?

(A) 45
(B) 66
(C) 90
(D) 121
(E) 132
Quick way to solve this.


The first ten terms are 3/1, 4/2, 5/3, ..., 11/9, 12/10

The product of these terms is (12*11*10*...*4*3)/10! = 12*11/2 = 66


66

just list out the number. then you will get it in a pretty fast way
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Re: The Infinite sequence [#permalink] New post 24 Jul 2008, 17:04
(11*12)/(1*2)=11*6 = 66 -> B
Re: The Infinite sequence   [#permalink] 24 Jul 2008, 17:04
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