It is currently 18 Oct 2017, 11:54

### GMAT Club Daily Prep

#### 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

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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# Sum of integers

Author Message
Intern
Joined: 23 Feb 2010
Posts: 21

Kudos [?]: 12 [0], given: 7

### Show Tags

07 Mar 2010, 15:45
00:00

Difficulty:

(N/A)

Question Stats:

100% (02:43) correct 0% (00:00) wrong based on 1 sessions

### HideShow timer Statistics

Please kindly provide explanations/shortcuts if possible. Thanks
Attachments

Q_13.JPG [ 67.1 KiB | Viewed 1848 times ]

Q_1.JPG [ 60.12 KiB | Viewed 1846 times ]

Kudos [?]: 12 [0], given: 7

Senior Manager
Joined: 22 Dec 2009
Posts: 356

Kudos [?]: 414 [1], given: 47

### Show Tags

07 Mar 2010, 15:53
1
KUDOS
gmatgg wrote:
Please kindly provide explanations/shortcuts if possible. Thanks

1.
If u notice, all rows starting from 2nd are mutliple of 1st row.
Sum of first row = 7*8/2 = 28 = k (lets say)

First row = k
Second row = -2k
third row = 3k
fourth row = -4k
fifth row = 5k
sixth row = -6k
seventh row = 7k

sum = k - 2k + 3k - 4k + 5k - 6k + 7k = 4k = 4*28 = 112 Ans
_________________

Cheers!
JT...........
If u like my post..... payback in Kudos!!

|For CR refer Powerscore CR Bible|For SC refer Manhattan SC Guide|

~~Better Burn Out... Than Fade Away~~

Kudos [?]: 414 [1], given: 47

Senior Manager
Joined: 22 Dec 2009
Posts: 356

Kudos [?]: 414 [0], given: 47

### Show Tags

07 Mar 2010, 16:02
gmatgg wrote:
Please kindly provide explanations/shortcuts if possible. Thanks

For 2nd ques, refer: sequence-88874.html?view-post=670848#p670848
_________________

Cheers!
JT...........
If u like my post..... payback in Kudos!!

|For CR refer Powerscore CR Bible|For SC refer Manhattan SC Guide|

~~Better Burn Out... Than Fade Away~~

Kudos [?]: 414 [0], given: 47

Intern
Joined: 23 Feb 2010
Posts: 21

Kudos [?]: 12 [0], given: 7

### Show Tags

07 Mar 2010, 17:21
Great explanation! thanks so much!!!!

Kudos [?]: 12 [0], given: 7

Manager
Joined: 13 Dec 2009
Posts: 248

Kudos [?]: 248 [0], given: 13

### Show Tags

15 Mar 2010, 09:35
Problem 1
We can say sum of postivie integers are (1+3+5+7)(1+2+3+4+5+6+7) = 16*7*8/2 = 448 (sum of numbers from 1 to n is n(n+1)/2)
and the sum of all the negative numbers = (2+4+6)(1+2+3+4+5+6+7) = 12*7*8/2 = 336
Subtract second from first and answer is 112
_________________

My debrief: done-and-dusted-730-q49-v40

Kudos [?]: 248 [0], given: 13

Manager
Joined: 13 Dec 2009
Posts: 248

Kudos [?]: 248 [0], given: 13

### Show Tags

15 Mar 2010, 09:44
(-1)^k+1*(1/2^k)

(-1)^k+1 says that every odd number will be positive and even number will be negative
so series will be like following
1/2-1/2^2+1/2^3-1/2^4....

=1/2(1+1/4+1/8+1/16+1/32) - 1/4(1+1/4+1/8+1/16+1/32)
=(1/2 - 1/4)(1+1/4+1/8+1/16+1/32)
=1/4*(32+16+8+4+1)/32
=1/4*61/32 = 61/128 =
1/4 = 32/128 multiply 32 in n and d
and 1/2 is 64/128 multiply 64 in n and d

so correct answer is it is between 1/4 to 1/2
_________________

My debrief: done-and-dusted-730-q49-v40

Kudos [?]: 248 [0], given: 13

Re: Sum of integers   [#permalink] 15 Mar 2010, 09:44
Display posts from previous: Sort by