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# Is x= a^b>0?

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 5255
GMAT 1: 800 Q59 V59
GPA: 3.82

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08 Dec 2017, 01:06
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Difficulty:

15% (low)

Question Stats:

80% (00:34) correct 20% (00:05) wrong based on 45 sessions

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[GMAT math practice question]

Is $$x= a^b>0$$?

1) $$a$$ is positive
2) $$b$$ is a prime number
[Reveal] Spoiler: OA

_________________

MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 3 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Expert Joined: 02 Aug 2009 Posts: 5774 Re: Is x= a^b>0? [#permalink] ### Show Tags 08 Dec 2017, 01:31 MathRevolution wrote: [GMAT math practice question] Is $$x= a^b>0$$? 1) $$a$$ is positive 2) $$b$$ is a prime number Pl correct the Question.. Most likely it should be is a^b>0.. 1) a is positive.. Whatever be the value of b, a^b>0 Suff 2) b is a prime number.. If it is 2, Ans will be YES always.. Otherwise it will depend on a. Insufficient A _________________ Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372 Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html GMAT online Tutor Senior Manager Joined: 05 Dec 2016 Posts: 260 Concentration: Strategy, Finance GMAT 1: 620 Q46 V29 Re: Is x= a^b>0? [#permalink] ### Show Tags 08 Dec 2017, 02:21 (1) a is positive, hence any value of b wqould give us positvie result Sufficient (2) b can take 2;3;7 etc a= -1 ; b=2 then x would be +ve a=-1 ; b=3 then x would be -ve Insufficient Answer A. Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 5255 GMAT 1: 800 Q59 V59 GPA: 3.82 Re: Is x= a^b>0? [#permalink] ### Show Tags 10 Dec 2017, 18:12 => 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 method is to modify the original condition and the question, and then recheck the question. Condition 1) If $$a > 0$$, then $$a^b>0$$. The answer is ‘yes’. So, this condition is sufficient. Condition 2) For $$a = 2, b = 2$$, we have $$a^b = 2^2 = 4 > 0$$, and the answer is ‘no’. For $$a = -2, b = 3$$, we have $$a^b = (-2)^2 = -8 < 0$$, and the answer is ‘yes’. So, condition 2) is not sufficient. Therefore, the answer is A. Normally, in problems which require 2 or more additional 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). Answer: A _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 3 month Online Course"
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Re: Is x= a^b>0?   [#permalink] 10 Dec 2017, 18:12
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