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A sequence is defined by an = (an-2)^2 - (an-1) for all terms n = 2, a

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A sequence is defined by an = (an-2)^2 - (an-1) for all terms n = 2, a [#permalink]

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A sequence is defined by \(a_{n} = (a_{n-2})^2 - (a_{n-1})\) for all terms n > 2, and \(a_1 = 2\) and \(a_2 = 1\). What is the value of \(a_7\)?
(A) – 64
(B) – 7
(C) 38
(D) 88
(E) 128


This is the final problem from a set of challenging GMAT Quant practice question. For the whole collection, as well as the OE for this question, see:
Challenging GMAT Math Practice Questions

Mike :-)

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A sequence is defined by an = (an-2)^2 - (an-1) for all terms n = 2, a [#permalink]

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I think the Answer is E.

Did it the brute force way - about 3 minutes
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Re: A sequence is defined by an = (an-2)^2 - (an-1) for all terms n = 2, a [#permalink]

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Put the values in equation.
a1 =2
a2=1
a3=3
a4=-2
a5=11
a6=-7
a7=128. Ans. E

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Re: A sequence is defined by an = (an-2)^2 - (an-1) for all terms n = 2, a [#permalink]

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New post 16 Mar 2017, 22:50
a1 =2;a2=1;a3=3;a4=-2;a5=11;a6=-7;a7=128, keep values to get the series , Hit Kudos if you liked it.
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Re: A sequence is defined by an = (an-2)^2 - (an-1) for all terms n = 2, a [#permalink]

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New post 17 Mar 2017, 01:17
a1 =2
a2=1
a3=3
a4=-2
a5=11
a6=-7
a7=128.

Option E
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Re: A sequence is defined by an = (an-2)^2 - (an-1) for all terms n = 2, a [#permalink]

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New post 07 Feb 2018, 02:54
a3= 4-1 = 3
a4= 1-3= -2
a5= 9 + 2= 11
a6= 4-11 = -7
a7= 121 + 7 = 128
Option E

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Re: A sequence is defined by an = (an-2)^2 - (an-1) for all terms n = 2, a [#permalink]

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New post 19 Mar 2018, 20:12
mikemcgarry wrote:
A sequence is defined by \(a_{n} = (a_{n-2})^2 - (a_{n-1})\) for all terms n > 2, and \(a_1 = 2\) and \(a_2 = 1\). What is the value of \(a_7\)?
(A) – 64
(B) – 7
(C) 38
(D) 88
(E) 128


This is the final problem from a set of challenging GMAT Quant practice question. For the whole collection, as well as the OE for this question, see:
Challenging GMAT Math Practice Questions

Mike :-)


a3 = (a1)^2 - (a2)
= 3

a4 = (a2)^2 - (a3)
= -2

a5 = (a3)^2 - (a4)
= 11

a6 = (a4)^2 - (a5)
= -7

a7 = (a5)^2 - (a6)

=128 = C the answer

thanks
Re: A sequence is defined by an = (an-2)^2 - (an-1) for all terms n = 2, a   [#permalink] 19 Mar 2018, 20:12
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