## sum of terms

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### sum of terms

by Sprite_TM » Wed Jun 10, 2009 5:30 pm
i think the question was like this

n= is the sequence number

a1 = 3 a2=5 an = a(n-2) - a(n-1)

what is the sum of 30 terms in the above sequence

a) 2
b) 3
c) 7
d) 8
e) 10

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### Re: sum of terms

by gmatbschool » Sun Jun 28, 2009 3:20 pm
Sprite_TM wrote:i think the question was like this

n= is the sequence number

a1 = 3 a2=5 an = a(n-2) - a(n-1)

what is the sum of 30 terms in the above sequence

a) 2
b) 3
c) 7
d) 8
e) 10

something is wrong with this question. ive spent close to an hour and neither of these answers make sense.

i started with writing out the terms to see if there was a pattern

a1 = 3
a2 = 5
a3 = a1 - a2
a4 = a2 - a3
a5 = a3 - a4

so a3 + a4 + a5 = a1 - a4
similarly a3 + ... a30 = a1 - a29
sum of all 30 is therefore 2a1 + a2 -a29

but finding a29 is the hardest part. also the sequence seems to be growing.

finally i got fed up and just put the sequence in excel and here are the results

a1 3
a2 5
a3 -2
a4 7
a5 -9
a6 16
a7 -25
a8 41
a9 -66
a10 107
a11 -173
a12 280
a13 -453
a14 733
a15 -1186
a16 1919
a17 -3105
a18 5024
a19 -8129
a20 13153
a21 -21282
a22 34435
a23 -55717
a24 90152
a25 -145869
a26 236021
a27 -381890
a28 617911
a29 -999801
a30 1617712
SUM 999812

OA pls? also pls check question.

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