127) what is a1

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ern5231: Master | Next Rank: 500 Posts; Posts: 226; Joined: Sun Aug 09, 2009 4:34 am

by ern5231 » Thu May 13, 2010 4:37 am

an=an-1-2an-2, n>=3. Given that a4=-12, a3=0. What is the value of a1? (note : subscripts are provided in bold)

a)3
b)4
c)5
d)6
e)8

Source: Beat The GMAT — Data Sufficiency | Thu May 13, 2010 4:37 am

nicktimoshenko: Senior | Next Rank: 100 Posts; Posts: 38; Joined: Thu Jan 21, 2010 12:03 am; Location: Kyiv, Ukraine; Thanked: 1 times; GMAT Score:610

by nicktimoshenko » Thu May 13, 2010 4:53 am

in first find a2

a4 = a3 - 2*a2

so a2 = (a3-a4)/2

after that find a1

first attempt 540 (Q48 V17)
second attempt 610 (q47 v27)
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arohan_sambyal: Junior | Next Rank: 30 Posts; Posts: 13; Joined: Thu May 13, 2010 4:16 am; Thanked: 1 times

by arohan_sambyal » Thu May 13, 2010 5:00 am

answer : 3
given a4=-12 and a3=0
a4=a3-2a2=>-12=0-2a2=>a2=6
a3=a2-2a1=>0=6-2a1=>6=2a1=>a1=3

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