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If S is the infinite sequence S1 = 6, S2 = 12, ..., Sn = [#permalink]
15 Feb 2008, 13:34

If S is the infinite sequence S1 = 6, S2 = 12, ..., Sn = Sn-1 + 6,..., what is the sum of all terms in the set {S13, S14, ..., S28}? 1,800 1,845 1,890 1,968 2,016 _________________

You tried your best and you failed miserably. The lesson is 'never try'. -Homer Simpson

Re: MGMAT - Sequence [#permalink]
15 Feb 2008, 13:57

bmwhype2 wrote:

If S is the infinite sequence S1 = 6, S2 = 12, ..., Sn = Sn-1 + 6,..., what is the sum of all terms in the set {S13, S14, ..., S28}? 1,800 1,845 1,890 1,968 2,016

sum = ((s13+s28)/2)*(28-13+1) = (s13+s28)*8 = (6 + 12*6+6+27*6)*8 = 6*8*(2+27+12)=6*8*41 = 1968 -> D

Re: MGMAT - Sequence [#permalink]
15 Feb 2008, 23:39

2

This post received KUDOS

bmwhype2 wrote:

If S is the infinite sequence S1 = 6, S2 = 12, ..., Sn = Sn-1 + 6,..., what is the sum of all terms in the set {S13, S14, ..., S28}? 1,800 1,845 1,890 1,968 2,016

Basically its just (n-1)*6+6 so S13 is (12)6+6 --> 78

S28 is (27)6+6 --> 168

Now there are 16 terms from 13 to 28 and the average is (168+78)/2 -->

Re: MGMAT - Sequence [#permalink]
19 Feb 2008, 07:36

GMATBLACKBELT wrote:

bmwhype2 wrote:

If S is the infinite sequence S1 = 6, S2 = 12, ..., Sn = Sn-1 + 6,..., what is the sum of all terms in the set {S13, S14, ..., S28}? 1,800 1,845 1,890 1,968 2,016

Basically its just (n-1)*6+6 so S13 is (12)6+6 --> 78

S28 is (27)6+6 --> 168

Now there are 16 terms from 13 to 28 and the average is (168+78)/2 -->

Now just 16*123 --> 1968

D

thanks. this is why i post questions that i know.

i did the entire problem via AP and then sum of AP, it took 3 minutes to solve. _________________

You tried your best and you failed miserably. The lesson is 'never try'. -Homer Simpson