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# To calculate

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Intern
Joined: 13 Feb 2009
Posts: 21
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To calculate [#permalink]  24 Feb 2009, 02:20
1
KUDOS
What is value of
1 - 1/2 - 1/6 - 1/12 - 1/20 - 1/30 - 1/42 - 1/56 ?
A. 1/7
B. 1/6
C. 1/8
D. 1/9
E. 1/10
SVP
Joined: 07 Nov 2007
Posts: 1828
Location: New York
Followers: 29

Kudos [?]: 541 [1] , given: 5

Re: To calculate [#permalink]  24 Feb 2009, 06:36
1
KUDOS
matematikconsultant wrote:
What is value of
1 - 1/2 - 1/6 - 1/12 - 1/20 - 1/30 - 1/42 - 1/56 ?
A. 1/7
B. 1/6
C. 1/8
D. 1/9
E. 1/10

(1 - 1/2) - (1/6+1/12) - (1/20+1/30) - (1/42+1/56)
=(1/2 - 1/4) - 1/12 - 1/24
=1/4 -1/8
=1/8
_________________

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VP
Joined: 05 Jul 2008
Posts: 1433
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Kudos [?]: 254 [0], given: 1

Re: To calculate [#permalink]  24 Feb 2009, 13:21
initially thought of LCM but quickly noticed that subtracting 1/6 from 1/2 gave 1/3 and then subtracting 1/12 gave 1/4

So the answer would be 1/8
Manager
Joined: 26 Dec 2008
Posts: 58
Schools: Booth (Admit R1), Sloan (Ding R1), Tuck (R1)
Followers: 2

Kudos [?]: 11 [0], given: 0

Re: To calculate [#permalink]  24 Feb 2009, 17:45
x2suresh wrote:
matematikconsultant wrote:
What is value of
1 - 1/2 - 1/6 - 1/12 - 1/20 - 1/30 - 1/42 - 1/56 ?
A. 1/7
B. 1/6
C. 1/8
D. 1/9
E. 1/10

(1 - 1/2) - (1/6+1/12) - (1/20+1/30) - (1/42+1/56)
=(1/2 - 1/4) - 1/12 - 1/24
=1/4 -1/8
=1/8

great approach, hats off to you. I am a newbie to the forum and have seen your posts in last 2-3 days -- you rock man!
Intern
Joined: 07 Oct 2008
Posts: 12
Followers: 0

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Re: To calculate [#permalink]  24 Feb 2009, 18:40
I am a new person on the form..

1-1/2-1/6-1/12-1/20-1/30-1/42-1/56
=>1/2-1/4-1/12-1/24
3/24=1/8
Intern
Joined: 13 Feb 2009
Posts: 21
Followers: 1

Kudos [?]: 2 [1] , given: 0

Re: To calculate [#permalink]  25 Feb 2009, 02:46
1
KUDOS
1- (1-1/2) - (1/2-1/3) - (1/3-1/4) - (1/4-1/5) - (1/5-1/6) -(1/6-1/7) - (1/7-1/8) = 1/8
Manager
Joined: 01 Jan 2008
Posts: 227
Schools: Booth, Stern, Haas
Followers: 1

Kudos [?]: 48 [0], given: 2

Re: To calculate [#permalink]  25 Feb 2009, 04:57
matematikconsultant wrote:
1- (1-1/2) - (1/2-1/3) - (1/3-1/4) - (1/4-1/5) - (1/5-1/6) -(1/6-1/7) - (1/7-1/8) = 1/8

I understood the approach you used, but how it helps to make it easier to solve?

thanks

Last edited by kbulse on 25 Feb 2009, 07:45, edited 1 time in total.
Intern
Joined: 13 Feb 2009
Posts: 21
Followers: 1

Kudos [?]: 2 [0], given: 0

Re: To calculate [#permalink]  25 Feb 2009, 05:37
Always, 1/(n(n+1)) = 1/n - 1/(n+1)
Re: To calculate   [#permalink] 25 Feb 2009, 05:37
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