AP-IV

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maihuna: Legendary Member; Posts: 1578; Joined: Sun Dec 28, 2008 1:49 amFollowed by: 9 members; Thanked: 82 times; GMAT Score:720

by maihuna » Sun Aug 16, 2009 3:48 am

If S1, S2, S3, ... , S50 are the sums of 20 terms of AP series whose first terms are 1, 2, 3, 4, ..., and whose common difference are 1,3,5,7,...what is the value of:

S1+S2+S3+..+S50

Charged up again to beat the beast

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Source: Beat The GMAT — Problem Solving | Sun Aug 16, 2009 3:48 am

Morgoth: Master | Next Rank: 500 Posts; Posts: 316; Joined: Mon Sep 22, 2008 12:04 amFollowed by: 1 member; Thanked: 36 times

Re: AP-IV

by Morgoth » Sun Aug 16, 2009 4:11 am

maihuna wrote:If S1, S2, S3, ... , S50 are the sums of 20 terms of AP series whose first terms are 1, 2, 3, 4, ..., and whose common difference are 1,3,5,7,...what is the value of:

S1+S2+S3+..+S50

S1, first term = 1, common difference = 1, no. of terms = 20

S2, first term = 2, common difference = 2, no. of terms = 20

so on and so forth.

sum of S1 = 20/2 [ 2*1 + (19)1]
sum of S2 = 20/2 [ 2*2 + (19)2]
sum of S3 = 20/2 [ 2*3 + (19)3]

so on and so forth

now you can easily identify the pattern

10[2+19] * [1+2+3+4+5.....+50]

10*21*1275 = 210*1275

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kaulnikhil: Master | Next Rank: 500 Posts; Posts: 338; Joined: Fri Apr 17, 2009 1:49 amFollowed by: 3 members; Thanked: 9 times

by kaulnikhil » Thu Aug 20, 2009 8:50 am

is it 487750??

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real2008: Legendary Member; Posts: 527; Joined: Thu May 01, 2008 12:06 am; Thanked: 7 times

by real2008 » Thu Aug 20, 2009 10:35 am

is it 500500

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