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Solve the Inequality [#permalink]
03 Aug 2009, 06:48
Question Stats:
0% (00:00) correct
0% (00:00) wrong based on 0 sessions
X > 1/X
Kindly help to solve ... Thanks in Advance.
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Re: Solve the Inequality [#permalink]
03 Aug 2009, 06:56
It would be helpful to know what the rest of the question is as well as the answer choices. Is this a data sufficiency?
As a statement, this is only true if X is positive. If X is negative, then that statement is false.
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Re: Solve the Inequality [#permalink]
03 Aug 2009, 08:57
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[EDIT - REMOVED BY USER]
Last edited by nplaneta on 17 Jun 2012, 17:43, edited 1 time in total.
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Re: Solve the Inequality [#permalink]
04 Aug 2009, 03:12
nplaneta wrote: X > 1 or -1 < X < 0 hey can you kindly explain in steps ,how you arrived at the answer..
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Re: Solve the Inequality [#permalink]
04 Aug 2009, 03:37
crejoc wrote: X > 1/X
Kindly help to solve ... Thanks in Advance. i will try and solve...Ian please comment x>1/x x-(1/x)>0 (x^2-1)/x > 0so EITHER 1/x > 0 and x^2 - 1 > 0 OR 1/x<0 and x^2 - 1 < 0 [ since (x^2-1)/x > 0 ...either both are positive or both are negative] Case 1: 1/x is greater than 0 then x > 0 and for x^2 - 1 > 0 ... x >1 or x>-1 so finally x>1 Cae 2: 1/x < 0 then x is less than 0 and for x^2 - 1 < 0 ..... x<-1 and x <1 so finally x < -1 PLEASE CORRECT MY APPROACH
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Re: Solve the Inequality [#permalink]
04 Aug 2009, 04:56
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apoorvasrivastva wrote: crejoc wrote: X > 1/X
Kindly help to solve ... Thanks in Advance. i will try and solve...Ian please comment x>1/x x-(1/x)>0 (x^2-1)/x > 0so EITHER 1/x > 0 and x^2 - 1 > 0 OR 1/x<0 and x^2 - 1 < 0 [ since (x^2-1)/x > 0 ...either both are positive or both are negative] Case 1: 1/x is greater than 0 then x > 0 and for x^2 - 1 > 0 ... x >1 or x>-1 so finally x>1 Cae 2: 1/x < 0 then x is less than 0 and for x^2 - 1 < 0 ..... x<-1 and x <1 so finally x < -1 PLEASE CORRECT MY APPROACH apoorvasrivastva, your approach is good for case 1, however for case 2 it will be -1 < x < 0 as nplaneta posted earlier. The reason being... (x^2 - 1) / x > 0 When x < 0 then: (x^2 - 1) < 0 [multiplying both sides by x (which is a -ve number, hence flipping the inequality)] x^2 < 1 now as we are considering x < 0 then, when we take square root of both side: sqrt(x^2) < sqrt(1) x < 1 [but x is a negative number so we need to multiply both sides by -1] -x > -1 x > -1 [left side became positive due to negative x and -1] and since the case is for x < 0 , therefore, the solution is -1 < x < 0 The solution for x > 1/x being: x>1 (when x>0) and -1 < x < 0 (when x<0) For reference and review of inequality principles, see basic-mathematical-principles post by HongHu in this forum [sorry I can't post the direct link, as the forum isn't allowing me cos I am new member] This exact problem is explained in this post as well. ~Cheers
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Re: Solve the Inequality [#permalink]
04 Aug 2009, 05:25
scarish wrote: For reference and review of inequality principles, see basic-mathematical-principles post by HongHu in this forum
Actually i got this problem from the very link you mentioned of HongHu's post, as it is not explained in detail there , i came up posting it in a separate thread... scarish wrote: case 2 it will be -1 < x < 0 as nplaneta posted earlier. When x < 0 then: (x^2 - 1) < 0 [multiplying both sides by x (which is a -ve number, hence flipping the inequality)] x^2 < 1 .......... x < 1 [but x is a negative number so we need to multiply both sides by -1]
I couldn't understand why you have to multiply x, you said to flip the inequality, but it is not flipped i think. Then why have to multiply by -1. sorry i don't get you. May be i am not comprehending it.. Could you explain clearly, the second case. i was also getting the same answer as apoorvasrivastva
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Re: Solve the Inequality [#permalink]
04 Aug 2009, 05:28
[EDIT - REMOVED BY THE USER]
Last edited by nplaneta on 17 Jun 2012, 19:13, edited 1 time in total.
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Re: Solve the Inequality [#permalink]
04 Aug 2009, 06:19
scarish wrote: Cae 2:
1/x < 0 then x is less than 0 and for x^2 - 1 < 0 ..... x<-1 and x <1
so finally x < -1
PLEASE CORRECT MY APPROACH apoorvasrivastva, your approach is good for case 1, however for case 2 it will be -1 < x < 0 as nplaneta posted earlier. The reason being... (x^2 - 1) / x > 0 When x < 0 then: (x^2 - 1) < 0 [multiplying both sides by x (which is a -ve number, hence flipping the inequality)] x^2 < 1 now as we are considering x < 0 then, when we take square root of both side: sqrt(x^2) < sqrt(1) x < 1 [but x is a negative number so we need to multiply both sides by -1] -x > -1 x > -1 [left side became positive due to negative x and -1] and since the case is for x < 0 , therefore, the solution is -1 < x < 0 The solution for x > 1/x being: x>1 (when x>0) and -1 < x < 0 (when x<0) For reference and review of inequality principles, see basic-mathematical-principles post by HongHu in this forum [sorry I can't post the direct link, as the forum isn't allowing me cos I am new member] This exact problem is explained in this post as well. ~Cheers[/quote] Thanks i realsed my mistake!!!Kudos given to u !!:)
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Re: Solve the Inequality [#permalink]
04 Aug 2009, 17:09
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scarish wrote: case 2 it will be -1 < x < 0 as nplaneta posted earlier. When x < 0 then: (x^2 - 1) < 0 [multiplying both sides by x (which is a -ve number, hence flipping the inequality)] x^2 < 1 .......... x < 1 [but x is a negative number so we need to multiply both sides by -1]
crejoc wrote: I couldn't understand why you have to multiply x, you said to flip the inequality, but it is not flipped i think. Then why have to multiply by -1. sorry i don't get you. May be i am not comprehending it.. Could you explain clearly, the second case. i was also getting the same answer as apoorvasrivastva Right, our equation was (x^2-1)/x > 0, so to get rid of x from the denominator you have to multiply both sides of the inequality by x, that is (x^2-1)*x/x > 0*x => (x^2-1)/x < 0 and the sign is flipped because x < 0. Multiplying both sides by x is what you do when we simply "cross-multiply" but we can only safely cross-multiply when it is equality but not in inequality. Secondly, upon solving our equation we arrived at: x < 1 Now x in this equation is -ve, since we are considering a case where x < 0. So if we leave it like x < 1 then the x in this case is not comparable with x in our solution for the case where x > 0. So in order to compare apples with apples, we need to get x to mean the same thing for both cases. Therefore, we multiply both sides by -1 (which flips the sign again) to arrive at: x > -1 Also, try picking few numbers in and outside the boundaries in the original equation, which may also help. crejoc wrote: Actually i got this problem from the very link you mentioned of HongHu's post, as it is not explained in detail there , i came up posting it in a separate thread...
On a separate note, I think if you have doubts on questions in a particular post then its better to post your doubts on that post itself, so as to keep all responses on the issue together, instead of starting the new post. I don't know if this is the policy on this forum or not, may be moderator can clarify?
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Re: Solve the Inequality [#permalink]
07 Aug 2009, 02:51
i think best approach in those 2 mins in GMAT will be to plug values.....just keep in mind that whenever it's about fraction....intervals should be 1<x; 0<x<1; -1<x<0; -1>x we find only applicable are 1<x and -1<x<0 i hope this helps
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Re: Solve the Inequality [#permalink]
09 Aug 2009, 05:24
bhushan252 wrote: i think best approach in those 2 mins in GMAT will be to plug values.....just keep in mind that whenever it's about fraction....intervals should be 1<x; 0<x<1; -1<x<0; -1>x
we find only applicable are 1<x and -1<x<0
i hope this helps Yes the faster the better, thanks for your approach...
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Re: Solve the Inequality [#permalink]
03 Apr 2011, 13:27
X - (1/X) > 0
=> (X2 -1)/X >0
=> X^2>1 , X>0 or X^2<1 , X<0
=> X>1 or -1<x<0
Last edited by Spidy001 on 03 Apr 2011, 18:46, edited 1 time in total.
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Re: Solve the Inequality [#permalink]
03 Apr 2011, 18:24
@Spidy001, I don't think you can multiply by x with the inequality sign intact without knowing the sign of x. See this solution : => x - 1/x > 0 => (x^2 -1)/x > 0 So the sign of numerator and denominator are same and if x > 0 then x^2 - 1 > 0 and if x < 0 then x^2 - 1 < 0 (x+1)(x-1) > 0 and (x - 1) (x+1) < 0 so x > 1 Or x < -1 AND -1 < x < 1 Hnce x > 1 OR -1 < x < 0
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Re: Solve the Inequality [#permalink]
03 Apr 2011, 18:38
Agreed. Good catch. subhashghosh wrote: @Spidy001, I don't think you can multiply by x with the inequality sign intact without knowing the sign of x.
See this solution :
=> x - 1/x > 0
=> (x^2 -1)/x > 0
So the sign of numerator and denominator are same and if x > 0 then x^2 - 1 > 0 and if x < 0 then x^2 - 1 < 0
(x+1)(x-1) > 0 and (x - 1) (x+1) < 0
so x > 1 Or x < -1 AND -1 < x < 1
Hnce x > 1 OR -1 < x < 0
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Re: Solve the Inequality
[#permalink]
03 Apr 2011, 18:38
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