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# PS: Sum

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SVP
Joined: 21 Jul 2006
Posts: 1512

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16 Nov 2008, 09:45
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This topic is locked. If you want to discuss this question please re-post it in the respective forum.

If the sum of the first 175 positive integers is x, what is the sum of the first 350 positive integers in terms of x?

a) $$(x+350)^2$$

b) $$x+175^2$$

c) $$2x + 175^2$$

d) $$4x+175$$

e) $$4x+350$$

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Current Student
Joined: 28 Dec 2004
Posts: 3345

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Location: New York City
Schools: Wharton'11 HBS'12

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16 Nov 2008, 10:05
i get C

basically I dont know but when I hit a road block, i pick small numbers and look for patterns..

so lets say x=4, then the sume of first 4 integers is 10..

now 350=175*2, so I pick 8 since 8=4*2..

sum of 8 integers is 36..which means 36=10*2+4^2
which is 2x+n^2

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SVP
Joined: 17 Jun 2008
Posts: 1534

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16 Nov 2008, 11:31
C.

The sum = x + x + (175 + 176 + 177 + ....till 175th number).
= 2x + 175*2*175/2

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Senior Manager
Joined: 21 Apr 2008
Posts: 269

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Location: Motortown

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16 Nov 2008, 20:43
scthakur wrote:
C.

The sum = x + x + (175 + 176 + 177 + ....till 175th number).
= 2x + 175*2*175/2

scthakur: Can you explain this in more detail.

Thx

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Director
Joined: 14 Aug 2007
Posts: 726

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16 Nov 2008, 23:32
[1+...+175] + (175 +1) +(175+2)....+(175+175)
We have 1+...+175 = x

x + 175*175 + [ 1+...+175]
x + 175^2 + x

C.

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SVP
Joined: 17 Jun 2008
Posts: 1534

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17 Nov 2008, 04:27
LiveStronger wrote:
scthakur wrote:
C.

The sum = x + x + (175 + 176 + 177 + ....till 175th number).
= 2x + 175*2*175/2

scthakur: Can you explain this in more detail.

Thx

Kudos [?]: 280 [0], given: 0

SVP
Joined: 21 Jul 2006
Posts: 1512

Kudos [?]: 1051 [0], given: 1

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17 Nov 2008, 08:50
OA is C.

Thanks for the explanation

Kudos [?]: 1051 [0], given: 1

Re: PS: Sum   [#permalink] 17 Nov 2008, 08:50
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