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Inequalities question

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by topspin360 » Mon May 05, 2014 9:50 am
Can someone show how to do this algebraically?I understand the answer intuitively but want to have the algebraic way in my back pocket.


Is x > 0?

A) x^6 > x^7
B) x^7 > x^8

Answer: B
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Source: — Data Sufficiency |

by GMATGuruNY » Mon May 05, 2014 10:47 am
topspin360 wrote:Can someone show how to do this algebraically?I understand the answer intuitively but want to have the algebraic way in my back pocket.


Is x > 0?

A) x^6 > x^7
B) x^7 > x^8

Answer: B
Both statements imply that x≠0.
Thus, the value of x raised to ANY EVEN POWER will be POSITIVE.
As a result, we can safely simplify the statements by dividing each side by x^(even power).

Statement 1: x� > x�
Dividing each side by x�, we get:
1 > x.
Thus, x can be any nonzero value less than 1.
Since it's possible that x=1/2 or that x=-1, INSUFFICIENT.

Statement 2: x� > x�
Dividing each side by x�, we get:
1/x > 1.
Since 1/x > 0, x>0.
SUFFICIENT.

The correct answer is B.
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by topspin360 » Mon May 05, 2014 12:06 pm
GMATGuruNY wrote:
topspin360 wrote:Can someone show how to do this algebraically?I understand the answer intuitively but want to have the algebraic way in my back pocket.


Is x > 0?

A) x^6 > x^7
B) x^7 > x^8

Answer: B
Both statements imply that x≠0.
Thus, the value of x raised to ANY EVEN POWER will be POSITIVE.
As a result, we can safely simplify the statements by dividing each side by x^(even power).

Statement 1: x� > x�
Dividing each side by x�, we get:
1 > x.
Thus, x can be any nonzero value less than 1.
Since it's possible that x=1/2 or that x=-1, INSUFFICIENT.

Statement 2: x� > x�
Dividing each side by x�, we get:
1/x > 1.
Since 1/x > 0, x>0.
SUFFICIENT.

The correct answer is B.
Haha Mitch I actually remember you showing me this solution for another problem. Have to remember the methods! Thanks!
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by feedrom » Tue May 06, 2014 9:56 am
Hi Mitch,

To the statement (2) x^7 > x^8, if I rearrange it by another way, it becomes confused:

x^7-x^8 >0
x^7(1-x) >0
From this, two cases will appear: x<1 or x>1

What's wrong with this way?
Thanks.
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