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If a1, a2, a3, ..., an, ... is a sequence such that an = 2n

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If a1, a2, a3, ..., an, ... is a sequence such that an = 2n [#permalink] New post 28 Mar 2012, 01:57
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If a1, a2, a3, ..., an, ... is a sequence such that an = 2n for all n>= 1, is ai greater than aj?

(1) i is add and j is even.

(2) i^2 > j^2
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Re: If a1, a2, a3, ..., an, ... is a sequence such that an = 2n [#permalink] New post 28 Mar 2012, 02:12
eybrj2 wrote:
If a1, a2, a3, ..., an, ... is a sequence such that an = 2n for all n>= 1, is ai greater than aj?

(1) i is add and j is even.

(2) i^2 > j^2


Since given that a_n = 2n, for all n\geq{1} then:
a_1=2*1=2;
a_2=2*2=4;
a_3=2*3=6;
a_4=2*4=8;
...

Basically we have a sequence of positive even numbers. Question asks whether a_i>a_j? So, it basically asks whether i>j?

(1) i is add and j is even. Not sufficient.

(2) i^2 > j^2 --> since i and j are both positive integers (they represent index numbers) then i>j. Sufficient.

Answer: B.

Hope it's clear.
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Re: If a1, a2, a3, ..., an, ... is a sequence such that an = 2n [#permalink] New post 31 Mar 2012, 11:08
Bunuel wrote:
eybrj2 wrote:
If a1, a2, a3, ..., an, ... is a sequence such that an = 2n for all n>= 1, is ai greater than aj?

(1) i is add and j is even.

(2) i^2 > j^2


Since given that a_n = 2n, for all n\geq{1} then:
a_1=2*1=2;
a_2=2*2=4;
a_3=2*3=6;
a_4=2*4=8;
...

Basically we have a sequence of positive even numbers. Question asks whether a_i>a_j? So, it basically asks whether i>j?

(1) i is add and j is even. Not sufficient.

(2) i^2 > j^2 --> since i and j are both positive integers (they represent index numbers) then i>j. Sufficient.

Answer: B.

Hope it's clear.

though answer will remain B
But
if i & j are index numbers and in sequence J>I
M i correct?
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Re: If a1, a2, a3, ..., an, ... is a sequence such that an = 2n [#permalink] New post 31 Mar 2012, 11:38
GMATD11 wrote:
Bunuel wrote:
eybrj2 wrote:
If a1, a2, a3, ..., an, ... is a sequence such that an = 2n for all n>= 1, is ai greater than aj?

(1) i is add and j is even.

(2) i^2 > j^2


Since given that a_n = 2n, for all n\geq{1} then:
a_1=2*1=2;
a_2=2*2=4;
a_3=2*3=6;
a_4=2*4=8;
...

Basically we have a sequence of positive even numbers. Question asks whether a_i>a_j? So, it basically asks whether i>j?

(1) i is add and j is even. Not sufficient.

(2) i^2 > j^2 --> since i and j are both positive integers (they represent index numbers) then i>j. Sufficient.

Answer: B.

Hope it's clear.

though answer will remain B
But
if i & j are index numbers and in sequence J>I
M i correct?


Not sure I understood your question, but i>j because it's given that i^2 > j^2.
_________________

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. NEW!!!

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set. NEW!!!


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Re: If a1, a2, a3, ..., an, ... is a sequence such that an = 2n [#permalink] New post 31 Mar 2012, 22:21
Vote for B

Given
So we have set of consicative number

& n>=1

{2,4,6,8,10.....}

is ai>aj

(A) i + j = even
o + o = e
e + e = e

so,
if (i>j) then ai>aj
if(i<j) then ai<aj
if (i=j) then aai=aj

data not suffficient

(B)

i^2 > j^2

we know for sure that i > j as n>=1 - i & j cannot be -ve

data sufficient
Re: If a1, a2, a3, ..., an, ... is a sequence such that an = 2n   [#permalink] 31 Mar 2012, 22:21
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