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PS: Sum

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PS: Sum [#permalink] New post 16 Nov 2008, 09:45
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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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Re: PS: Sum [#permalink] New post 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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Re: PS: Sum [#permalink] New post 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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Re: PS: Sum [#permalink] New post 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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Re: PS: Sum [#permalink] New post 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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Re: PS: Sum [#permalink] New post 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



Reading it late.....already explained by alpha_plus_gama.
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Re: PS: Sum [#permalink] New post 17 Nov 2008, 08:50
OA is C.

Thanks for the explanation
Re: PS: Sum   [#permalink] 17 Nov 2008, 08:50
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