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M11 #1

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I3igDmsu
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M11 #1 [#permalink] New post   30 Jun 2009, 19:23
What is \(1 + 2 + ... + 98\) ?
A 4750
B 4763
C 4790
D 4801
E 4851

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OA=E


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benpack03
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Re: M11 #1 [#permalink] New post   01 Jul 2009, 09:40
I got E

98+1=99, 97+2=99, 96+3.......There are 49 of these additions (98/2) so:

49*99=4851
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nitya34
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Re: M11 #1 [#permalink] New post   01 Jul 2009, 09:55
Its AP with a=1,d=1,n=98

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

its E
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I3igDmsu
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Re: M11 #1 [#permalink] New post   02 Jul 2009, 04:50
Figured out another way:

S = (n(Last+First))/2
n= (First-Last) + 1

n=98
S=(98(99))/2 = 4851 so E.

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Re: M11 #1 [#permalink]
02 Jul 2009, 04:50
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Bunuel
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