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# The Sum of first N consecutive odd integers is N^2. What is

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The Sum of first N consecutive odd integers is N^2. What is [#permalink]

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02 Aug 2006, 17:34
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The Sum of first N consecutive odd integers is N^2. What is the sum of all odd integers between 13 and 39 inclusive.

A. 351
B. 364
C. 410
D. 424
E. 450
[Reveal] Spoiler: OA

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Manager
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02 Aug 2006, 17:40
B....

1- 11 are the first 6 odd intergers; sum = 6^2 = 36

1- 39 are the first 20 odd intergers; sum = 400

sum of odd nos frm 13- 39 = 400-36 = 364

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02 Aug 2006, 19:44
Use AP and you woudn't need the supplied formula:

a(1) = 13; a(n) = a(1) + (n-1)d = 39 (where d =2).
Sum = n/2[a(1) + a(n)] = 364, where n = 14
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02 Aug 2006, 23:24
Number of odd numbers from 1-39 = 39+1/2 = 20
Sum = 400

Number of odd numbers between = 1-12 = (12 - 1 +1)/2 = 6
sum = 36

Sum between 13-39 inclusive = 400 - 36 = 364

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03 Aug 2006, 09:03
I also used AP without using N^2 condition. For me, sometimes, it is easeir to just stick to basics and do the calcs. This way, at least, I know I will arrive at the correct answer sooner or later...

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04 Aug 2006, 00:41
I would suggest using the formula, for the simple reason that instead of an AP the problem might refer to some fancy formula for an unknown sequence of numbers.

Anyway here we go...

For odd integers till 39, number of integers = 20
Sum = 20^2 = 400
For odd integers till (but not including 13), number of integers = 6
Sum = 6^2 = 36
Difference = 364 = B

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Re: PS : Number Prop. [#permalink]

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04 Aug 2006, 06:43
ghantark wrote:
The Sum of first N consecutive odd integers is N^2. What is the sum of all odd integers between 13 and 39 inclusive.

351
364
410
424
450

OA later

i never knew this formulea that sum of n consecutive odd integers = n^2.
however, we can use regular formula:
the sum of consecutive odd integers from 13-39 = n (L-s)/2 = [{(39-13)/2} +1][39-13] = 14 x 26 = 364

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Re: PS : Number Prop. [#permalink]

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05 Aug 2006, 01:03
Professor wrote:
ghantark wrote:
The Sum of first N consecutive odd integers is N^2. What is the sum of all odd integers between 13 and 39 inclusive.

351
364
410
424
450

OA later

i never knew this formulea that sum of n consecutive odd integers = n^2.
however, we can use regular formula:
the sum of consecutive odd integers from 13-39 = n (L-s)/2 = [{(39-13)/2} +1][39-13] = 14 x 26 = 364

It can be derived from the regular formula for A.P.

1+3+5+....=n/2[2.1+(n-1).2] = n^^2 !!
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Zooroopa

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Re: PS : Number Prop. [#permalink]

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05 Aug 2006, 15:37
ghantark wrote:
The Sum of first N consecutive odd integers is N^2. What is the sum of all odd integers between 13 and 39 inclusive.

351
364
410
424
450

OA later

Sum of odd integers from 1 to 39 inclusive = 20^2=400
sum of odd integers from 1 to 11 inclusive = 6^2 = 36
therfore sum of odd integers between 13 and 39 incl = 400 -36 = 364

Hence B

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Re: The Sum of first N consecutive odd integers is N^2. What is [#permalink]

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07 Jun 2012, 01:58
1
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I method -
the sum of odd numbers till 39(inclusive) =n^2=20^2

the sum of odd numbers till 11 (inclusive) =n^2=6^2

20^2 -6^2=(20-6)(20+6)=26*14=364

================

II method
n* (a1+an)/2
n=((39-13)/2)+1=14

(39+13)/2* 14=26*14=364
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Re: The Sum of first N consecutive odd integers is N^2. What is [#permalink]

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21 Jan 2015, 18:44
ghantark wrote:
The Sum of first N consecutive odd integers is N^2. What is the sum of all odd integers between 13 and 39 inclusive.

A. 351
B. 364
C. 410
D. 424
E. 450

from the question I see that it ask about the sum of odd numbers between 13 and 39 not from 13 to 39 therefore, 13 and 39 are not includes in the sum.So, there are 20 odd numbers from 1 to 39 so 20^2 = 400 and from 1 to 13 there are 7 odd numbers so it will be 7^2=49
400-49=351

if I am no right what will be the answer if the question was ask about the sum of odd numbers from 13 to 39
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Re: The Sum of first N consecutive odd integers is N^2. What is [#permalink]

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21 Jan 2015, 19:59
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Hi All,

These types of "sum of a sequence" questions can be approached in a number of different ways - you can use various formulas or you can approach this prompt by pattern-matching and minimizing the math that you have to do. For this question, you can use "bunching"....

We're dealing with a sequence of CONSECUTIVE ODD INTEGERS: 13 to 39, inclusive. We're asked for the SUM of this group.

1) Start with the sum of the smallest and the biggest: 13 + 39 = 52
2) Now look at the 'next smallest' and the 'next biggest': 15 + 37 = 52

From this, you can see that you're just going to be adding up a bunch of 52s. We DO have to check to see if there's a "middle" term in this sequence that doesn't get "bunched" though. To determine if that middle term exists, we just have find the last few 52s in the group....

21 and 31
23 and 29
25 and 27

Now we have proof that there is no middle term. We have 7 bunches of 52.

7(52) = 364

[Reveal] Spoiler:
B

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Special Offer: Save $75 + GMAT Club Tests Free Official GMAT Exam Packs + 70 Pt. Improvement Guarantee www.empowergmat.com/ ***********************Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!*********************** Kudos [?]: 3402 [0], given: 172 SVP Status: The Best Or Nothing Joined: 27 Dec 2012 Posts: 1853 Kudos [?]: 2616 [1], given: 193 Location: India Concentration: General Management, Technology WE: Information Technology (Computer Software) Re: The Sum of first N consecutive odd integers is N^2. What is [#permalink] ### Show Tags 22 Jan 2015, 03:13 1 This post received KUDOS 23a2012 wrote: ghantark wrote: The Sum of first N consecutive odd integers is N^2. What is the sum of all odd integers between 13 and 39 inclusive. A. 351 B. 364 C. 410 D. 424 E. 450 from the question I see that it ask about the sum of odd numbers between 13 and 39 not from 13 to 39 therefore, 13 and 39 are not includes in the sum.So, there are 20 odd numbers from 1 to 39 so 20^2 = 400 and from 1 to 13 there are 7 odd numbers so it will be 7^2=49 400-49=351 if I am no right what will be the answer if the question was ask about the sum of odd numbers from 13 to 39 Its asking sum of odd integers between 13 and 39 inclusive, so we require to minus sum of odd integers up to 11 inclusive $$= 20^2 - 6^2 = 26 * 14 = 364$$ Answer = B _________________ Kindly press "+1 Kudos" to appreciate Kudos [?]: 2616 [1], given: 193 Manager Status: Kitchener Joined: 03 Oct 2013 Posts: 96 Kudos [?]: 26 [0], given: 144 Location: Canada Concentration: Finance, Finance GPA: 2.9 WE: Education (Education) Re: The Sum of first N consecutive odd integers is N^2. What is [#permalink] ### Show Tags 22 Jan 2015, 16:23 ghantark wrote: The Sum of first N consecutive odd integers is N^2. What is the sum of all odd integers between 13 and 39 inclusive. A. 351 B. 364 C. 410 D. 424 E. 450 Thank you for EMPOWERgmatRichC and PareshGmat But I still do not understand what will be the answer if the question was ask about the sum of odd numbers from 13 to 39 and why we do not includ 13? _________________ Click +1 Kudos if my post helped Kudos [?]: 26 [0], given: 144 EMPOWERgmat Instructor Status: GMAT Assassin/Co-Founder Affiliations: EMPOWERgmat Joined: 19 Dec 2014 Posts: 9962 Kudos [?]: 3402 [0], given: 172 Location: United States (CA) GMAT 1: 800 Q51 V49 GRE 1: 340 Q170 V170 Re: The Sum of first N consecutive odd integers is N^2. What is [#permalink] ### Show Tags 22 Jan 2015, 17:37 Hi 23a2012, My explanation (and everyone else's too) includes 13 and 39 in the sum. The prompt uses the phrase "ALL odd integers between 13 and 39, INCLUSIVE." This means that we have to INCLUDE 13 and 39 and all of the explanations do that. GMAT assassins aren't born, they're made, Rich _________________ 760+: Learn What GMAT Assassins Do to Score at the Highest Levels Contact Rich at: Rich.C@empowergmat.com # Rich Cohen Co-Founder & GMAT Assassin Special Offer: Save$75 + GMAT Club Tests Free
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Re: The Sum of first N consecutive odd integers is N^2. What is [#permalink]

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Re: The Sum of first N consecutive odd integers is N^2. What is [#permalink]

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23 Aug 2016, 21:27
ghantark wrote:
The Sum of first N consecutive odd integers is N^2. What is the sum of all odd integers between 13 and 39 inclusive.

A. 351
B. 364
C. 410
D. 424
E. 450

The numbers to be added are 13,15,...39. These are in Arithmetic progression with difference as 2.

a1 (first term) = 13
d = 2
N (number of terms)= $$\frac{(39-13)}{2} + 1 = 14$$
Sum = $$\frac{N}{2} *(2*a1 + (N-1)*d)$$
= $$\frac{14}{2} *(2*13 + (14-1)2)$$
= 364

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Re: The Sum of first N consecutive odd integers is N^2. What is [#permalink]

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18 Sep 2016, 11:54
what if the question was exlusive instead of inclusive ?
can you please show me the difference ?
I have tried to do it but i got confused should i remove 1 and 39 or just one of them
Thank you :D

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Re: The Sum of first N consecutive odd integers is N^2. What is   [#permalink] 18 Sep 2016, 11:54
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