gmat prep xy

Riggz · Posted: Wed May 07, 2008 3:15 pm Post subject: gmat prep xy

if R= 1 + 2XY + (X^2)(Y^2), what is the value of XY?

1) R=0

2) X> 0

TIA!

zacharyz · Posted: Wed May 07, 2008 4:06 pm Post subject:

I will go with (A)

Question gives:
R = 1 + 2xy + x^2*y^2
Factor this to get
R = (1 + xy)(1 + xy) = (1 + xy)^2

Statement 1:
If R = 0, then either of the factors have to equal zero (and note that there is only one factor)
that means that xy = -1

Sufficient -> you know the answer.

Statement 2:
x is positive
Insufficient - this doesn't give you any indication of R or Y (or a real number for x)

No need to proceed any further and put them together, (A) was sufficient and (B) was not... there the answer is (A)