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

Question on a CAT problem - number properties

by CaitlinL Mon Aug 04, 2008 11:22 pm

I don't understand this question (or the answers provided). Can anyone help me? Thanks!

If |x| + |y| = -x - y and xy does not equal 0, which of the following must be true?

The |x| + |y| on the left side of the equation will always add the positive value of x to the positive value of y, yielding a positive value. Therefore, the -x and the -y on the right side of the equation must also each yield a positive value. The only way for -x and -y to each yield positive values is if both x and y are negative.

(A) FALSE: For x + y to be greater than zero, either x or y has to be positive.

(B) TRUE: Since x has to be negative and y has to be negative, the sum of x and y will always be negative.

(C) UNCERTAIN: All that is certain is that x and y have to be negative. Since x can have a larger magnitude than y and vice-versa, x - y could be greater than zero.

(D) UNCERTAIN: All that is certain is that x and y have to be negative. Since x can have a larger magnitude than y and vice versa, x - y could be less than zero.

(E) UNCERTAIN: As with choices (C) and (D), we have no idea about the magnitude of x and y. Therefore, x2 - y2 could be either positive or negative.

Another option to solve this problem is to systematically test numbers. With values for x and y that satisfy the original equation, observe that both x and y have to be negative. If
x = -4 and y = -2, we can eliminate choices (A) and (C). Then, we might choose numbers such that y has a greater magnitude than x, such as x = -2 and y = -4. With these values, we can eliminate choices (D) and (E).

The correct answer is B.
RonPurewal
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Joined: Tue Aug 14, 2007 8:23 am
 

by RonPurewal Thu Sep 04, 2008 7:39 am

hi -

could you please post the actual answer choices?
you've got the explanations for those choices, showing why they're true/false/undecided, but the stuff you've copied and pasted (or typed out) doesn't include the actual answer choices themselves.

thanks.