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TrebleMaker85
- Newbie | Next Rank: 10 Posts
- Posts: 3
- Joined: Mon Nov 23, 2009 11:29 am•Followed by: 1 member
- Location: Los Angeles, CA
- GMAT Score:710
IMO - E
what is OA ?
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Two variables
Source: Beat The GMAT — Data Sufficiency |
Official answer is E.
Examples:
1. x = 0, y = 1 satisfies (1) and (2) and yields "No"
2. x = 3, y = 4 satisfies (1) and (2) and yields "Yes"
Obviously this is true, but can anyone think of a way to solve this without a) graphing all inequalities or 2) trial and error?
Examples:
1. x = 0, y = 1 satisfies (1) and (2) and yields "No"
2. x = 3, y = 4 satisfies (1) and (2) and yields "Yes"
Obviously this is true, but can anyone think of a way to solve this without a) graphing all inequalities or 2) trial and error?
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Stuart@KaplanGMAT
- GMAT Instructor
- Posts: 3225
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Let's start by simplifying the original by multiplying both sides by y² (we can safely do so because we know that y² is positive):TrebleMaker85 wrote:Is x² - y > 1/y² where y is non-zero?
(1) x < y
(2) x² < y²
Thoughts?
Is x²y² - y^3 > 1
(arg, can't seem to cut and paste superscripted numbers!)
(1) x < y
If x and y are big positive numbers, y^3 will dominate x²y² and the left side will be negative; if x and y are big negative numbers, x²y² and (-y^3) will both be positive and the left side will be a big positive number: insufficient.
(2) x² < y²
Tells us that |x| < |y|, but nothing about the signs of x and y. If we pick really big values with different signs we can generate both "yes" and "no" answers.
Together: combining doesn't help us at all, since we have the same generalizations from both: choose E.
* * *
Moral of the story: there are lots of DS number property questions on which you can reason out the correct answer, but picking numbers is often quicker. To get a 700+ score on the GMAT you must be able to effectively pick numbers, there just isn't time to apply esoteric principles to every question.
Stuart Kovinsky | Kaplan GMAT Faculty | Toronto
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