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What is the largest prime factor of the sum of the series of

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What is the largest prime factor of the sum of the series of [#permalink] New post 11 Dec 2005, 03:02
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What is the largest prime factor of the sum of the series of consecutive integers from 1 to 30, inclusive?

A. 31
B. 17
C. 13
D. 11
E. 5

Please show your work.
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 [#permalink] New post 11 Dec 2005, 03:05
sum of consecutive numbers. .
n*(n+1)/2

so. (30*31)/2

hence 31.. largest prime factor..at 5 in the morning. .
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 [#permalink] New post 13 Dec 2005, 14:21
Got 31 quickly but not as efficiently as bewakof.
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 [#permalink] New post 13 Dec 2005, 15:26
lhotseface wrote:
Got 31 quickly but not as efficiently as bewakof.


Bewakoof is not all that bewakoof after all :)
(bewakoof = dumb in Hindi/Urdu)
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Re: PS Arithmetic progression [#permalink] New post 13 Dec 2005, 22:32
GMATT73 wrote:
What is the largest prime factor of the sum of the series of consecutive integers from 1 to 30, inclusive?

A. 31
B. 17
C. 13
D. 11
E. 5

Please show your work.


Answer is A

Sum = (# of integers) (average) = (30-1+1) (31/2) = 465 = 5*3*31

bewakoof, what are you doing up at 5am in the morning?



:lol:
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Re: PS Arithmetic progression [#permalink] New post 13 Dec 2005, 23:05
TeHCM wrote:
GMATT73 wrote:
What is the largest prime factor of the sum of the series of consecutive integers from 1 to 30, inclusive?

A. 31
B. 17
C. 13
D. 11
E. 5

Please show your work.


Answer is A

Sum = (# of integers) (average) = (30-1+1) (31/2) = 465 = 5*3*31

bewakoof, what are you doing up at 5am in the morning?

5am? Give that man a 700!
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Re: PS Arithmetic progression [#permalink] New post 14 Dec 2005, 00:09
GMATT73 wrote:
What is the largest prime factor of the sum of the series of consecutive integers from 1 to 30, inclusive?

A. 31
B. 17
C. 13
D. 11
E. 5

Please show your work.


sum = n/2(a+l), where a is the first term, l is the last term, and n is the number of terms in the series.
= 30/2(1+30)
= 15 * 31
31 is a prime number, and factor of 15*31, hence A.
Re: PS Arithmetic progression   [#permalink] 14 Dec 2005, 00:09
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