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# What is 1 + 2 + 3 ...98? 4750 4763 4790 4801 4851

Author Message
CEO
Joined: 21 Jan 2007
Posts: 2598
Location: New York City
What is 1 + 2 + 3 ...98? 4750 4763 4790 4801 4851   [#permalink]

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28 Oct 2007, 12:09
00:00

Difficulty:

(N/A)

Question Stats:

100% (02:01) correct 0% (00:00) wrong based on 3 sessions

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What is 1 + 2 + 3 ...98?

4750
4763
4790
4801
4851

What is the logic of your approach towards this question?

--== Message from the GMAT Club Team ==--

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Director
Joined: 11 Jun 2007
Posts: 866

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28 Oct 2007, 12:23
bmwhype2 wrote:
What is 1 + 2 + 3 ...98?

4750
4763
4790
4801
4851

What is the logic of your approach towards this question?

i get 4851 using the summation formula for arithmetic progression

Sn = n/2 (2a + (n-1) d

n = # of terms = 98
a = value of first term = 1
d = difference between terms = 1 (since consecutive)

Sn = 98/2 ( 2*1 + (98-1)(1))
= 49 (2+97) = 4851
VP
Joined: 08 Jun 2005
Posts: 1133

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28 Oct 2007, 12:24
(98*99)/2 = 4,851

This is true for every 1+2+3 ... n

sum =(n*(n+1))/2

Director
Joined: 08 Jun 2007
Posts: 546

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28 Oct 2007, 12:26
bmwhype2 wrote:
What is 1 + 2 + 3 ...98?

4750
4763
4790
4801
4851

What is the logic of your approach towards this question?

My logic is to remember formula for arithmetic progression.

Sn = n/2 [ 2a + (n-1) d]

= 98 * 99 /2 = 4851
Manager
Joined: 02 Aug 2007
Posts: 142

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28 Oct 2007, 14:50
Does anyone has any practice problems testing this forumla?
Director
Joined: 01 May 2007
Posts: 771

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28 Oct 2007, 14:53
1
Check the PR book. This is a pretty standard GMAT formula you should know for the test. And to break it down in simple terms:

(How many #s in set/2) TIMES (First Number in set + Last Number in set) so..

(98/2) * (99) = 4851

Things to watch out for is the question...how many #s in the set. It just so happens we started with 1 in this case. But what if I said 13 to 98, inclusive. How many integers then...answer?
Manager
Joined: 02 Aug 2007
Posts: 142

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28 Oct 2007, 15:02
13-98 is the range with d = 1

Sum of numbers in this sequence:
= 4675
Manager
Joined: 19 Aug 2007
Posts: 161

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28 Oct 2007, 15:13
another way to solve it is:
1+98 = 99
2+97 = 99
3+96 = 99

so you will have 49 values that are all equal to 99
so 49*99 = 4851
CEO
Joined: 21 Jan 2007
Posts: 2598
Location: New York City

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28 Oct 2007, 22:00
jimjohn wrote:
another way to solve it is:
1+98 = 99
2+97 = 99
3+96 = 99

so you will have 49 values that are all equal to 99
so 49*99 = 4851

this is the way i solved this.

OA is E.
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 4803
Location: Singapore

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29 Oct 2007, 00:35
This is an arithmetric progression where first term = 1, last term = 98, number of terms = 98. Then sum of this progression = 98/2 (1 + 98) = 4851

ans D
Non-Human User
Joined: 09 Sep 2013
Posts: 9449
Re: What is 1 + 2 + 3 ...98? 4750 4763 4790 4801 4851   [#permalink]

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21 Mar 2017, 19:23
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--== Message from the GMAT Club Team ==--

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This discussion does not meet community quality standards. It has been retired.

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To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

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Re: What is 1 + 2 + 3 ...98? 4750 4763 4790 4801 4851 &nbs [#permalink] 21 Mar 2017, 19:23
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