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What is 1/(1)*(2) + 1/(2)*(3) + 1/(3)*(4) + /1(4)*(5) +....

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Re: What is 1/(1)*(2) + 1/(2)*(3) + 1/(3)*(4) + /1(4)*(5) +.... [#permalink]

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New post 02 Aug 2015, 04:12
sashibagra wrote:

I have doubt about it!
Ain't it time consuming and how much time did you consumed to solve this math your way? ( Just curious so would be better if you mention time needed to solve your way of thinking. I appreaciate thinking about possibilities but if during GMAT exam we are under time constrain so students always go after those techniques that helps them to pick the right answer in best possible time.

Thanks.


Not really. If you end up going for longer methods in solving GMAT questions, then you have surely missed out on certain information or methods that could have saved you some useful seconds (every second counts in GMAT!). You can see it this way: if lets say after looking at this question, your 1st instinct was to solve it by the fraction adding method (by taking the common LCM etc) or your 1st instinct was to solve it by converting fractions to decimals, then surely you have missed something. GMAT quant questions always have elegant and less time intensive solutions.

I do agree that this method of writing the given sequence in a particular way wont come straightaway but analyzing a particular question to see whether there is a method that will save time will help you in gaining those useful extra seconds. For me, I looked at the question and gave it 5-10 seconds as to how to tackle it. Converting to fractions was very straightforward and thus time consuming. It took me another 15 seconds to figure out what could be the relation (1/an+1 - 1/an etc) and finally another 20-25 seconds to write down the given sequence in the most productive manner. In total the total time was ~1 minute.

If you wouldve gone the fraction to decimal route, you would have spent some crucial seconds in calculating whats the decimal representation of 1/42 or 1/56 etc.

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Re: What is 1/(1)*(2) + 1/(2)*(3) + 1/(3)*(4) + /1(4)*(5) +.... [#permalink]

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New post 03 Aug 2015, 13:01
RIght off the bat, you can see that 1/2 + 1/4 = 3/4. So, the answer is already greater that choices A), B) or C). Based on the acscending nature of the answer choices, we can conclude that 46/55 is less than 9/10. But just using some reasoning, we are already at .75, 1/90 is a bit more than .01, 1/30 is a bit more than .03, 1/20 is .05. Adding these into .75, we are already at .84. Knowing the other 3 terms will give us approximately another .05, we can safely choose answer choice E.
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Re: What is 1/(1)*(2) + 1/(2)*(3) + 1/(3)*(4) + /1(4)*(5) +.... [#permalink]

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Re: What is 1/(1)*(2) + 1/(2)*(3) + 1/(3)*(4) + /1(4)*(5) +.... [#permalink]

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New post 20 Dec 2016, 04:10
Hi, solved it in 1 min 32 sec.
Pattern: n/n+1 ==> 9/9+1 Answer E

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Re: What is 1/(1)*(2) + 1/(2)*(3) + 1/(3)*(4) + /1(4)*(5) +.... [#permalink]

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New post 20 Dec 2016, 04:10
Hi, solved it in 1 min 32 sec.
Pattern: n/n+1 ==> 9/9+1 Answer E

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What is 1/(1)*(2) + 1/(2)*(3) + 1/(3)*(4) + /1(4)*(5) +.... [#permalink]

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New post 20 Dec 2016, 06:14
goodyear2013 wrote:
What is 1/(1)*(2) + 1/(2)*(3) + 1/(3)*(4) + /1(4)*(5) + /1(5)*(6) + 1/(6)*(7) + 1/(7)*(8) + 1/(8)*(9) + 1/(9)*(10)?

2/5
3/5
7/10
46/55
9/10

Hi,
I want to know what is the best way to solve this question in 2min, please.


Hi Experts, Please correct if my approach is wrong.

1/2 + 1/6 +1/12 = .5 + .16 + .08 = .74 > 7/10, So choices A, B & C are out. Now, as 1/90 is multiple of 10, denominator will also be a multiple of 10 and hence choice D is also out.

Thus, Answer is choice E

Thanks

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Re: What is 1/(1)*(2) + 1/(2)*(3) + 1/(3)*(4) + /1(4)*(5) +.... [#permalink]

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New post 03 Dec 2017, 05:59
This is from the NCERT text book of math for class 11 in indian school.



PareshGmat wrote:
GyanOne wrote:
Tn = nth term of the series = 1/(n)(n+1) = (n+1-n)/(n)(n+1) = 1/n - 1/(n+1)

If you add all the terms of this series (to n terms), you get a sum of 1 - 1/(n+1) as all the other terms cancel out.
So sum of 9 terms = 1 - 1/10 = 9/10.
Option (E).


This is excellent. How long it took to derive the formula?

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Re: What is 1/(1)*(2) + 1/(2)*(3) + 1/(3)*(4) + /1(4)*(5) +....   [#permalink] 03 Dec 2017, 05:59

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