Math questions from any Manhattan Prep GMAT Computer Adaptive Test.
Guest
 
 

If x is not equal to 0, is |x| less than 1?

by Guest Wed Jun 20, 2007 5:09 pm

I think the answer to this question should be A only
After squaring both sides you get;
x^2 > 1
So 1<x<-1 and the answer to the question stem is always NO..

Can Manhattan Instructors pls confirm if this is correct....
Thanks
Image
StaceyKoprince
ManhattanGMAT Staff
 
Posts: 9355
Joined: Wed Oct 19, 2005 9:05 am
Location: Montreal
 

MGMAT CAT DS

by StaceyKoprince Thu Jun 21, 2007 8:27 pm

if x is not equal to zero, is lxl < 1?

Statement 1: [x/lxl] < x
You can't just square both sides because there are two possibilities here: that x/lxl = 1 or that x/lxl = -1. You've just lost one of the possibilities (-1) by squaring both sides. That's why you're only getting one answer, the one that equates to the positive case. See below for the correct manipulations.

You can multiply both sides by lxl to get x < xlxl (Note: don't have to switch signs b/c absolute value of anything is never negative.)

Then, you can divide the plain x on the right side BUT you have to remember there are two possibilities: when x is positive and when x is negative.
if x is positive, then x/x < lxl or 1 < lxl, in which case we answer the question no.
if x is negative, then x/x > lxl or 1 > lxl, in which case we answer the question yes.

These answers contradict, so the statement is insufficient.
Stacey Koprince
Instructor
Director, Content & Curriculum
ManhattanPrep