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#D3

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GHong14 Master | Next Rank: 500 Posts
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#D3

Post Fri Jun 03, 2011 11:21 am
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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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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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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