Welcome! Check out our free B-School Guides to learn how you compare with other applicants.

## DS: number properties

tagged by: GMAT Kolaveri

Joined
07 Dec 2011
Posted:
18 messages
Followed by:
1 members
Test Date:
TBD
Target GMAT Score:
750
GMAT Score:
600
DS: number properties Wed Apr 25, 2012 8:10 am
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])

If n is an integer and x(raised to the power)n - x(raised to the power)-n = 0, what is the value of x ?
(1) x is an integer.
(2) n ≠ 0

_________________

Regards,

Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!

### GMAT/MBA Expert

Stuart Kovinsky GMAT Instructor
Joined
08 Jan 2008
Posted:
3156 messages
Followed by:
461 members
Thanked:
1433 times
GMAT Score:
800
Wed Apr 25, 2012 9:33 am

If n is an integer and x(raised to the power)n - x(raised to the power)-n = 0, what is the value of x ?
(1) x is an integer.
(2) n ≠ 0
We know that:

x^n - x^(-n) = 0
or
x^n = x^(-n)

and we want to find the value for x.

Let's start by analyzing the question stem, something too many test takers fail to do.

A negative exponent is the same as 1 over that exponent. In other words:

x^(-n) = 1/(x^n)

So, we can rewrite the equation as:

x^n = 1/(x^n)

Now let's ask ourselves: when can a number equal its reciprocal? We answer ourselves: only if that number is 1 or -1.

So, we now know that x^n = 1 or -1

We ask ourselves another question: when will x^n = 1 or -1? Feeling particularly brilliant today, we have no trouble answering ourselves that could happen if:

1) n=0 (since anything raised to the exponent 0 = 1);
2) x = 1 (since 1 raised to any exponent is still 1); or
3) x = -1 (since -1 raised to any exponent is -1 or 1).

To the statements!

(1) x is an integer.

No help here at all; x could still be -1 or 1 (or anything else if n=0); insufficient, eliminate A and D.

(2) n doesn't equal 0

This statement eliminates one of the 3 possibilities (that n=0 and x could be anything), but x could still be 1 or -1; insufficient, eliminate B.

Since neither statement is sufficient alone, we have to look at the statements together:

(2) eliminates the n=0 possibility, but both -1 and 1 satisfy statements (1) and (2). Therefore, we don't have a specific value for x; still insufficient, eliminate C. Choose E!

_________________
Stuart Kovinsky, B.A. LL.B.
Toronto Office
1-800-KAP-TEST
www.kaptest.com

Thanked by: shantanu290
Free GMAT Practice Test under Proctored Conditions! - Find a practice test near you or live and online in Kaplan's Classroom Anywhere environment. Register today!
jrakhe Just gettin' started!
Joined
22 Apr 2012
Posted:
17 messages
Wed Apr 25, 2012 5:11 pm
We are given that
x^n - x^(-n) = 0

Which can re-written as:
x^2n - 1 = 0

1) x is an integer
if x=1 and n=1
1^2 - 1 = 0 (true)
if x=2 and n=1
2^2 - 1 = 0 (not true)
So choice 1 is not sufficient

2) n ≠ 0
In this case also we can use the values used in choice 1
So choice 2 is not sufficient

### Best Conversation Starters

1 varun289 40 topics
2 guerrero 21 topics
3 sana.noor 20 topics
4 killerdrummer 19 topics
5 sanaa.rizwan 14 topics
See More Top Beat The GMAT Members...

### Most Active Experts

1 Brent@GMATPrepNow

GMAT Prep Now Teacher

203 posts
2 GMATGuruNY

The Princeton Review Teacher

140 posts
3 Jim@StratusPrep

Stratus Prep

100 posts
4 Anju@Gurome

Gurome

99 posts