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M19-05

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M19-05 [#permalink]

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New post 16 Sep 2014, 01:05
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A
B
C
D
E

Difficulty:

  35% (medium)

Question Stats:

66% (00:59) correct 34% (00:58) wrong based on 149 sessions

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Expert Post
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Re M19-05 [#permalink]

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New post 16 Sep 2014, 01:05
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Official Solution:


Is \(1+(x+x^2)+(x^3+x^4)+...+(x^9+x^{10}) \gt 0\)?

(1) \(x \lt -1\). From this statement it follows that \(x+x^2 \gt 0\), \(x^3+x^4 \gt 0\), ..., \(x^9+x^{10} \gt 0\), so the sum is also more than zero. Sufficient.

(2) \(x^2 \gt 2\). This statement implies that \(x \lt -\sqrt{2}\) or \(x \gt \sqrt{2}\). Even if \(x\) itself is negative then still as above: \(x+x^2 \gt 0\), \(x^3+x^4 \gt 0\), ..., \(x^9+x^{10} \gt 0\), so the sum is also more than zero. Sufficient.


Answer: D
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Re: M19-05 [#permalink]

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New post 26 Mar 2017, 09:38
Bunuel wrote:
Is \(1 + x + x^2 + ... + x^{10}\) positive?


(1) \(x \lt -1\)

(2) \(x^2 \gt 2\)




So essentially we need to figure whether x is between 0-1 or not. is that correct? Thank you.

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Re: M19-05 [#permalink]

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New post 02 Apr 2017, 16:20
Strange question...
Wouldn't the answer be positive regardless what the statement is?
two statements add no value to the answer to this question...

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Re: M19-05 [#permalink]

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Expert's post
Bunuel wrote:
Is \(1 + x + x^2 + ... + x^{10}\) positive?


(1) \(x \lt -1\)

(2) \(x^2 \gt 2\)


Hi,

The Q in the present format may not require any statement.

\(1 + x + x^2 + ... + x^{10}\)

1) If x is POSITIVE, it will always be positive.

2) If x is NEGATIVE and x is between 0 and -1.
\(1 + x + x^2 + ... + x^{10}= (1+x)+(x^2+x^3)+....(x^8+x^9)+x^{10}\)
Here x is between 0 and 1..
So 1+x will be positive.
\(x^2+x^3\) will have x^2 as positive and x^3 as negative but the numeric value of lower powers will be greater. Example (1/2)^2=1/4 whereas (1/2)^3 is 1/8....
Similarly all other brackets too would be positive and thus total equation will be positive..
3) If x is NEGATIVE and <-1..
\(1 + x + x^2 + ... + x^{10}= 1+(x+x^2)+(x^3+x^4)+.....+(x^9+x^{10})\)..
Here each bracket will be POSITIVE as higher power mean higher value and each bracket has higher EVEN power.
Again overall POSITIVE.

Bunuel, the Q will be correct if we remove 1 from equation and Q becomes
Is \(x + x^2 + ... + x^{10}\) positive?
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Re: M19-05 [#permalink]

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New post 04 Apr 2017, 08:43
chetan2u wrote:
Bunuel wrote:
Is \(1 + x + x^2 + ... + x^{10}\) positive?


(1) \(x \lt -1\)

(2) \(x^2 \gt 2\)


Hi,

The Q in the present format may not require any statement.

\(1 + x + x^2 + ... + x^{10}\)

1) If x is POSITIVE, it will always be positive.

2) If x is NEGATIVE and x is between 0 and -1.
\(1 + x + x^2 + ... + x^{10}= (1+x)+(x^2+x^3)+....(x^8+x^9)+x^{10}\)
Here x is between 0 and 1..
So 1+x will be positive.
\(x^2+x^3\) will have x^2 as positive and x^3 as negative but the numeric value of lower powers will be greater. Example (1/2)^2=1/4 whereas (1/2)^3 is 1/8....
Similarly all other brackets too would be positive and thus total equation will be positive..
3) If x is NEGATIVE and <-1..
\(1 + x + x^2 + ... + x^{10}= 1+(x+x^2)+(x^3+x^4)+.....+(x^9+x^{10})\)..
Here each bracket will be POSITIVE as higher power mean higher value and each bracket has higher EVEN power.
Again overall POSITIVE.

Bunuel, the Q will be correct if we remove 1 from equation and Q becomes
Is \(x + x^2 + ... + x^{10}\) positive?


Thank you. I'm revising the question.
_________________

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Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
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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. ,11 Mixed Questions, 12 Fresh Meat

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., 11 New DS set.


What are GMAT Club Tests?
Extra-hard Quant Tests with Brilliant Analytics

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Re: M19-05 [#permalink]

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New post 06 Apr 2017, 09:23
If I'm understanding this correctly, you just need to know if x is between 0 and -1.

Posted from my mobile device

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Re: M19-05 [#permalink]

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New post 31 Jul 2017, 04:46
Bunuel wrote:
chetan2u wrote:
Bunuel wrote:
Is \(1 + x + x^2 + ... + x^{10}\) positive?


(1) \(x \lt -1\)

(2) \(x^2 \gt 2\)


Hi,

The Q in the present format may not require any statement.

\(1 + x + x^2 + ... + x^{10}\)

1) If x is POSITIVE, it will always be positive.

2) If x is NEGATIVE and x is between 0 and -1.
\(1 + x + x^2 + ... + x^{10}= (1+x)+(x^2+x^3)+....(x^8+x^9)+x^{10}\)
Here x is between 0 and 1..
So 1+x will be positive.
\(x^2+x^3\) will have x^2 as positive and x^3 as negative but the numeric value of lower powers will be greater. Example (1/2)^2=1/4 whereas (1/2)^3 is 1/8....
Similarly all other brackets too would be positive and thus total equation will be positive..
3) If x is NEGATIVE and <-1..
\(1 + x + x^2 + ... + x^{10}= 1+(x+x^2)+(x^3+x^4)+.....+(x^9+x^{10})\)..
Here each bracket will be POSITIVE as higher power mean higher value and each bracket has higher EVEN power.
Again overall POSITIVE.

Bunuel, the Q will be correct if we remove 1 from equation and Q becomes
Is \(x + x^2 + ... + x^{10}\) positive?


Thank you. I'm revising the question.


The question is still not reversed; please remove 1...thanks
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Re: M19-05 [#permalink]

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New post 20 Aug 2017, 00:12
it is a GP
1+X^2+X^3....+X^10= (X^11-1)/(X-1)
RHS is always positive if |X|>1

1- Statement: 1..says X<-1, means |X|>1...therefore RHS is positive always: Statement is sufficient
2- Statement: 2..says X^2>2, means |X|>1...therefore RHS is positive always: Statement is sufficient

hence Answer is D

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Re: M19-05   [#permalink] 20 Aug 2017, 00:12
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