Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

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

Show Tags

03 Aug 2015, 14:01

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.
_________________

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

Show Tags

20 Dec 2016, 07:14

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.

Concentration: International Business, General Management

GPA: 3.64

WE: Business Development (Energy and Utilities)

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

Show Tags

03 Dec 2017, 06:59

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?