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If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0 [#permalink]
12 Dec 2012, 13:40
st 1 says 1/(k-1)>0 that means k-1>0 that means k>1 that means k is +ve and thus 1/k>0 ... suff
st2 says 1/(k+1) >0 that means k+1>0 i.e k>-1 that means k can -ve fraction or k is positive . if k is -neg fraction 1/k <0 and if k is +ve 1/k >0 so insuff
Thus the answer is A.
What is the OA?
This is lovely. When I took the GMAT Prep and encountered this q, I thought that flipping denominator would require switching the inequality too. Clearly that is not the case. This approach is so much more efficient than is plugging in numbers.
Re: If k <> 0, 1, or -1, is 1/k > 0 ? [#permalink]
25 Aug 2013, 05:32
It must be A.
Again it is not mentioned that k is an integer.
1) 1/k-1>0 => k can be any value greater than 1 to satisfy this equation hence 1/k >0; sufficient. 2) 1/1+k>0 => if k=0.5 then 1/k >0 it holds true, but if k=-0.5 this equation still holds whereas, 1/k<0; therefore not sufficient.
--It's one thing to get defeated, but another to accept it.
Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.
If k does not equal 0, 1 or -1, is 1/k >0
(1) 1/(k-1) > 0 (2) 1/(k+1) > 0
When it comes to inequality DS questions, 2 things are important at all times. First is square. Secondly, when range of que includes range of con, the con is sufficient. Modify the original condition and the question. Multiply k^2 on the both equations and the sign of inequality doesn't change as k^2 is still a positive integer even when it's multiplied. There is 1 variable(k), which should match with the number of equations. So you need 1 equation, for 1) 1 equation, for 2) q equation, which is likely to make D the answer. For 1), multiply (k-1)^2 on the both equations, they become k-1>0, k>1. The range of que includes the range of con, which is sufficient. For 2), multiply (k-1)^2 on the both equations, they become k-1>0, k>1. The range of que doesn't include the range of con, which is not sufficient Therefore, the answer is A.
-> For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.