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Kaplan GMAT 800 DS solution wrong

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Kaplan GMAT 800 DS solution wrong

by nimesh » Mon Mar 23, 2009 12:17 pm
Hi

This question comes from the 08-09 edition of Kaplan GMAT 800 (DS-Straight Math no. 16, p364).

If a, b and c are distinct nonzero numbers, is

((a+b)^2 * (b-c))/((a-b)^3 * (b-c)^3) greater or equal to zero?

1) a>b
2) b>c

Solution given:
Find out if the expression is non-negative. Simplify expression to (a+b)^2 / ((a-b)^3 * (b-c)^2). The signs of the squared terms must be positive whilst the sign of the cubed term is unknown. But if the latter is not negative, expression is not negative ie. if a>b. Statement 1 tells you, so answer is (A).
However statement 1 could be true and cubed term could still be negative eg. a=-2, b=-3. So I believe the correct answer is (E)!

Agreed?
Cheers
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Source: — Data Sufficiency |
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Re: Kaplan GMAT 800 DS solution wrong

by 2ndShot » Mon Mar 23, 2009 12:41 pm
nimesh wrote: However statement 1 could be true and cubed term could still be negative eg. a=-2, b=-3. So I believe the correct answer is (E)!
No, if a>b then a-b must be positive, since
if both are positive the smaller number is subtracted from the bigger number,
if a is positive and b is negative, a negative number is subtracted, i.e. a positive number is added,
AND if a smaller negative number is subtracted from a greater negative number its positive again (like in your case a-b=-2-(-3)=-2+3=1)

So the book seems correct. Probably you looked at the wrong term. Its (a-b)^3.

Cheers
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