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A sequence is such that, an=1/n-1/(n+1). What is the sum of

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Director
Joined: 17 Oct 2005
Posts: 928
A sequence is such that, an=1/n-1/(n+1). What is the sum of [#permalink]

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22 Mar 2006, 18:27
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

A sequence is such that, an=1/n-1/(n+1). What is the sum of the
first 100 terms?
VP
Joined: 29 Dec 2005
Posts: 1341

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22 Mar 2006, 19:31
joemama142000 wrote:
A sequence is such that, an=1/n-1/(n+1). What is the sum of the first 100 terms?

hmm... interesting..

a1=1/1-1/2
a2=1/2 - 1/3
a3=1/3 - 1/4
........
.........
.........
a100 = 1/100 - 1/101

so, sum = 1 - 1/101 = 100/101
Director
Joined: 04 Jan 2006
Posts: 922

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22 Mar 2006, 22:10
good solution professor..

I like the simplicity.. For some reason I started using Arithematic Progression formulaes... And it did not work with the formulaes..

any ideas why?
Manager
Joined: 12 Feb 2006
Posts: 71

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23 Mar 2006, 12:54
Professor wrote:
joemama142000 wrote:
A sequence is such that, an=1/n-1/(n+1). What is the sum of the first 100 terms?

hmm... interesting..

a1=1/1-1/2
a2=1/2 - 1/3
a3=1/3 - 1/4
........
.........
.........
a100 = 1/100 - 1/101

so, sum = 1 - 1/101 = 100/101

prof, can you please explain the part
1- 1/101= 100/101
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The greater the sacrifice, the greater the Victory

Director
Joined: 24 Oct 2005
Posts: 572
Location: NYC

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23 Mar 2006, 15:15
Angela780 wrote:
Professor wrote:
joemama142000 wrote:
A sequence is such that, an=1/n-1/(n+1). What is the sum of the first 100 terms?

hmm... interesting..

a1=1/1-1/2
a2=1/2 - 1/3
a3=1/3 - 1/4
........
.........
.........
a100 = 1/100 - 1/101

so, sum = 1 - 1/101 = 100/101

prof, can you please explain the part
1- 1/101= 100/101

a1 + a2 + a3 + a4 +....

all the terms cancel out from a2 to a99 because of the alternative negative and postive signs
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Success is my only option, failure is not -- Eminem

Manager
Joined: 12 Feb 2006
Posts: 71

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23 Mar 2006, 15:28
Ok, thank you for the explanation.
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The greater the sacrifice, the greater the Victory

VP
Joined: 29 Dec 2005
Posts: 1341

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23 Mar 2006, 19:50
Angela780 wrote:
Professor wrote:
joemama142000 wrote:
A sequence is such that, an=1/n-1/(n+1). What is the sum of the first 100 terms?

hmm... interesting..

a1=1/1-1/2
a2=1/2 - 1/3
a3=1/3 - 1/4
........
.........
.........
a100 = 1/100 - 1/101

so, sum = 1 - 1/101 = 100/101

prof, can you please explain the part
1- 1/101= 100/101

= 1- 1/101 = (1) - (1)/(101) = (101-1)/(101) = 100/101
Director
Joined: 17 Oct 2005
Posts: 928

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24 Mar 2006, 17:37
oa is 100/101 great work prof
Manager
Joined: 21 Mar 2006
Posts: 131

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28 Mar 2006, 17:28
Professor wrote:
Angela780 wrote:
Professor wrote:
joemama142000 wrote:
A sequence is such that, an=1/n-1/(n+1). What is the sum of the first 100 terms?

hmm... interesting..

a1=1/1-1/2
a2=1/2 - 1/3
a3=1/3 - 1/4
........
.........
.........
a100 = 1/100 - 1/101

so, sum = 1 - 1/101 = 100/101

prof, can you please explain the part
1- 1/101= 100/101

= 1- 1/101 = (1) - (1)/(101) = (101-1)/(101) = 100/101

I'm not sure why you take 1-1/101... could you explain why? I read that it cancels out but I still don't see how it cancels?
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A.P.

Manager
Joined: 21 Mar 2006
Posts: 89

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28 Mar 2006, 22:10
Quote:
1 - 1/101

I also don't exactly understand how you get to the above. Please explain in more detail. Thanks.
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5043
Location: Singapore

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29 Mar 2006, 07:12
Charlie45 wrote:
Quote:
1 - 1/101

I also don't exactly understand how you get to the above. Please explain in more detail. Thanks.

a1 = 1/1 - 1/2
a2 = 1/2 - 1/3
a3 = 1/3 - 1/4

Just taking sum of a1 + a2 + a3, we have

1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 = 1/1 - 1/4.

You can see that the middle terms all cancel out leaving you with the first term and last term.

Just one more term, in case you're not convinced...

a4 = 1/4 - 1/5

so a1 + a2 + a3 + a4 + a5 = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 = 1/1 - 1/5
Manager
Joined: 21 Mar 2006
Posts: 89

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29 Mar 2006, 09:59
Thanks ywilfred, very clever solution.
Manager
Joined: 21 Mar 2006
Posts: 131

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29 Mar 2006, 21:12
ur a genious!
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A.P.

Manager
Joined: 20 Nov 2005
Posts: 188
Location: USA

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30 Mar 2006, 22:10
Professor wrote:
joemama142000 wrote:
A sequence is such that, an=1/n-1/(n+1). What is the sum of the first 100 terms?

hmm... interesting..

a1=1/1-1/2
a2=1/2 - 1/3
a3=1/3 - 1/4
........
.........
.........
a100 = 1/100 - 1/101

so, sum = 1 - 1/101 = 100/101

all i can say that you are super genius........
Re: sequence more   [#permalink] 30 Mar 2006, 22:10
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