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To calculate

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Intern
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To calculate [#permalink] New post 24 Feb 2009, 02:20
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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
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Re: To calculate [#permalink] New post 24 Feb 2009, 06:36
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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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Re: To calculate [#permalink] New post 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
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Re: To calculate [#permalink] New post 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!
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Re: To calculate [#permalink] New post 24 Feb 2009, 18:40
I am a new person on the form..

Got the same answer
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
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Re: To calculate [#permalink] New post 25 Feb 2009, 02:46
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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
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Re: To calculate [#permalink] New post 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
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Re: To calculate [#permalink] New post 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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