[GMAT math practice question]
$$Is\ x^y<0?$$
1) x is positive
2) y is a prime number
Is x^y <0?
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Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question.
Question:
x^y <0
=> x < 0 and y is an odd integer.
Condition 1)
Since x > 0, x^y > 0, regardless of the value of y.
Since 'no' is also a unique answer by CMT (Common Mistake Type) 1, condition 1) is sufficient.
Condition 2)
If x = -1 and y = 3, x^y = (-1)^3 = -1 < 0, and the answer is 'yes'.
If x = -1 and y = 2, x^y = (-1)^2 = 1 > 0, and the answer is 'no'.
Condition 2) is not sufficient since the question does not have a unique answer.
Therefore, A is the answer.
Answer: A
Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question.
Question:
x^y <0
=> x < 0 and y is an odd integer.
Condition 1)
Since x > 0, x^y > 0, regardless of the value of y.
Since 'no' is also a unique answer by CMT (Common Mistake Type) 1, condition 1) is sufficient.
Condition 2)
If x = -1 and y = 3, x^y = (-1)^3 = -1 < 0, and the answer is 'yes'.
If x = -1 and y = 2, x^y = (-1)^2 = 1 > 0, and the answer is 'no'.
Condition 2) is not sufficient since the question does not have a unique answer.
Therefore, A is the answer.
Answer: A
Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
Math Revolution
The World's Most "Complete" GMAT Math Course!
Score an excellent Q49-51 just like 70% of our students.
[Free] Full on-demand course (7 days) - 100 hours of video lessons, 490 lesson topics, and 2,000 questions.
[Course] Starting $79 for on-demand and $60 for tutoring per hour and $390 only for Live Online.
Email to : [email protected]