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#D3

This topic has 2 member replies
GHong14 Master | Next Rank: 500 Posts
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#D3

Post Fri Jun 03, 2011 11:21 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    The sequence a1 a2....an is such that an = (an-1 + an-2)/2 for all n>=3. If a3=4 and a5=20, what is the value of a6.

    12
    16
    20
    24
    28

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    Post Fri Jun 03, 2011 11:35 am
    let a(n) means a subscript n
    a(n) = (a(n-1) + a(n-2))/2
    a3=4 and a5=20
    if n=5; 20 = (a(4) + 4)/2 => a(4) = 36
    Now a6 = (a4+a5)/2 = (20+36)/2 = 28
    IMO E

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    breakkgmat Junior | Next Rank: 30 Posts Default Avatar
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    Post Sat Jun 04, 2011 1:03 am
    We need to compute a4 in order to get the value of a6.
    so, a5=a4+a3/2 or 20=a4+4/2 or 40=a4+4 or, a4=36.

    So, a6 = a5+a4/2 = 20+36/2 = 56/2 = 28.Ans E.

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