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# A sequence of numbers is given by the rule an = (an – 1)^2. What is a5

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Math Expert
Joined: 02 Sep 2009
Posts: 49985
A sequence of numbers is given by the rule an = (an – 1)^2. What is a5  [#permalink]

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29 Oct 2017, 01:56
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15% (low)

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87% (00:49) correct 13% (01:11) wrong based on 59 sessions

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A sequence of numbers is given by the rule $$a_n = (a_{n – 1})^2$$. What is $$a_5$$?

(1) $$a_1 = –1$$
(2) $$a_3 = 1$$

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Math Expert
Joined: 02 Aug 2009
Posts: 6963
Re: A sequence of numbers is given by the rule an = (an – 1)^2. What is a5  [#permalink]

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29 Oct 2017, 04:36
Bunuel wrote:
A sequence of numbers is given by the rule $$a_n = (a_{n – 1})^2$$. What is $$a_5$$?

(1) $$a_1 = –1$$
(2) $$a_3 = 1$$

$$a_n = (a_{n – 1})^2$$ tells us that each term is linked to another term..
so knowing ANY number in the sequence can give us the value of ALL terms in the sequence ..

we are looking for $$a_5$$, so knowing any other term will help us in knowing it...

Both statements give us one value of a term, so EACH statement is sufficient

D
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1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

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Re: A sequence of numbers is given by the rule an = (an – 1)^2. What is a5  [#permalink]

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29 Oct 2017, 10:57
Both Statements are sufficient.

If a1 is given we can calculate a2,a3,a4 and then a5.

If a3 is given, we can calculate a4 and then a5.

IMO:D
Re: A sequence of numbers is given by the rule an = (an – 1)^2. What is a5 &nbs [#permalink] 29 Oct 2017, 10:57
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# A sequence of numbers is given by the rule an = (an – 1)^2. What is a5

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