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Is X between 0 and 1 ? (1) x^2 is less than x (2) x^3 is

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Is X between 0 and 1 ? (1) x^2 is less than x (2) x^3 is [#permalink] New post 04 Nov 2010, 18:49
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Is X between 0 and 1 ?

(1) x^2 is less than x
(2) x^3 is positive

I am curious how to rephrase Statement 1 using inequalities. I rewrote it as x^2 - x < 0 , which then gives me x(x-1) < 0. If x < 0 and x < 1 then 0>x<1. Wouldnt this statement be insufficient? or am i writing that dual inequality incorrectly?
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Re: GMAT Quant Rev 2nd Ed - DS 76 [#permalink] New post 04 Nov 2010, 18:55
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jscott319 wrote:
Is X between 0 and 1 ?

1) x^2 is less than x
2) x^3 is positive

I am curious how to rephrase Statement 1 using inequalities. I rewrote it as x^2 - x < 0 , which then gives me x(x-1) < 0. If x < 0 and x < 1 then 0>x<1. Wouldnt this statement be insufficient? or am i writing that dual inequality incorrectly?



x(x-1) < 0 gives you the solution 0 < x < 1.

check out the link below for the explanation:
http://gmatclub.com/forum/inequalities-trick-91482.html#p804990
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Re: GMAT Quant Rev 2nd Ed - DS 76 [#permalink] New post 04 Nov 2010, 19:02
Hmm i think i am more confused after reading that the first time through....

I think i am missing the reason why the inequality sign for x < 0 should actually bex > 0. I determined x< 1 because i set the inequality of x - 1 < 0 and after subtracting from both sides give me x < 1 . What is different about x < 0 becoming x > 0 ?
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Re: GMAT Quant Rev 2nd Ed - DS 76 [#permalink] New post 04 Nov 2010, 19:14
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jscott319 wrote:
Hmm i think i am more confused after reading that the first time through....

I think i am missing the reason why the inequality sign for x < 0 should actually bex > 0. I determined x< 1 because i set the inequality of x - 1 < 0 and after subtracting from both sides give me x < 1 . What is different about x < 0 becoming x > 0 ?


x(x - 1) < 0 is not the same as x <0 and (x - 1)< 0

When I multiply two terms, the result is negative if and only if one of them is negative and the other is positive. When I multiply x with (x - 1), the result x(x - 1) will be negative (less than 0) in two cases:

Case I: x < 0 (x is negative) but (x - 1) > 0 (x - 1 is positive)
(x - 1) > 0 implies x > 1
But this is not possible. x cannot be less than 0 and greater than 1 at the same time.

Case II: x > 0 (x is positive) but (x - 1) < 0 (x - 1 is negative)
(x - 1) < 0 implies x < 1
This will happen when x lies between 0 and 1. i.e. when 0 < x < 1.

The link gives you the shortcut of solving inequalities of this type.
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Last edited by VeritasPrepKarishma on 04 Nov 2010, 19:18, edited 1 time in total.
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Re: GMAT Quant Rev 2nd Ed - DS 76 [#permalink] New post 04 Nov 2010, 19:15
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jscott319 wrote:
Is X between 0 and 1 ?

1) x^2 is less than x
2) x^3 is positive

I am curious how to rephrase Statement 1 using inequalities. I rewrote it as x^2 - x < 0 , which then gives me x(x-1) < 0. If x < 0 and x < 1 then 0>x<1. Wouldnt this statement be insufficient? or am i writing that dual inequality incorrectly?


Is x between 0 and 1?

Is 0<x<1?

(1) x^2 is less than x --> x^2<x --> x(x-1)<0:

Multiples must have opposite signs:
x<0 and x-1>0, or x>1 --> no solution (x can not be simultaneously less than zero and more than 1);
x>0 and x-1<0, or x<1 --> 0<x<1;

So x(x-1)<0 holds true when 0<x<1. Sufficient.

For alternate approach check "How to solve quadratic inequalities": x2-4x-94661.html#p731476

(2) x^3 is positive --> x^3>0 just tells us that x is positive. Not sufficient.

Answer: A.
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Re: GMAT Quant Rev 2nd Ed - DS 76 [#permalink] New post 04 Nov 2010, 19:27
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Ok I see it now! I was not taking into consideration the 2 cases that you've just made clear for me. Now I see how x(x-1) < 0 must become 0<x<1 . Thanks guys!
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Arithmetic operation [#permalink] New post 21 Feb 2011, 10:12
is x between 0 and 1?
1. x^2 is less than x
2. x^3 is positive

I answer C considering x could be negative or positive but option 2 ensures x is positive. Please help what is the wrong with me.
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Re: Arithmetic operation [#permalink] New post 21 Feb 2011, 10:19
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Merging similar topics.

Baten80 wrote:
is x between 0 and 1?
1. x^2 is less than x
2. x^3 is positive

I answer C considering x could be negative or positive but option 2 ensures x is positive. Please help what is the wrong with me.


In (1) as x^2<x then x can not be negative, because if it is then we would have x^2>0>x.
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Re: GMAT Quant Rev 2nd Ed - DS 76 [#permalink] New post 21 Feb 2011, 10:22
Q: Is 0<x<1?

1.
x^2<x
x^2-x<0
x(x-1)<0

Means;
case I:
x<0 and x-1>0=>x>1
OR
case II:
x>0 and x-1<0=>x<1
case I is impossible. x can't be greater than 1 and less than 0 at the same time.

Thus; only case II is valid and x>0 and x<1
In other words; 0<x<1
Sufficient.

2.x^3 is +ve.
if x=0.1; x^3=.001(a positive value); 0<x<1
if x=2; x^3=8(a positive value); but x>1
Not sufficient.

Ans: "A"
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Re: Is X between 0 and 1 ? (1) x^2 is less than x (2) x^3 is [#permalink] New post 07 Jun 2012, 17:44
I have a question is this. Why have we considered both the options.

x(x-1)<0:

Multiples must have opposite signs:
1. x<0 and x-1>0, or x>1 --> no solution (x can not be simultaneously less than zero and more than 1);
2. x>0 and x-1<0, or x<1 --> 0<x<1;

In the link to the post when we find the roots of the quadratic equation and if we know the sign is "<" we can directly right the roots as "root 1" < x < "root 2". The same way in this case also there are 2 roots 0 and 1 so we can directly write it this way. 0<x<1. Why do we have to consider case 1 also.

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Re: Is X between 0 and 1 ? (1) x^2 is less than x (2) x^3 is [#permalink] New post 08 Jun 2012, 02:33
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rggoel9 wrote:
I have a question is this. Why have we considered both the options.

x(x-1)<0:

Multiples must have opposite signs:
1. x<0 and x-1>0, or x>1 --> no solution (x can not be simultaneously less than zero and more than 1);
2. x>0 and x-1<0, or x<1 --> 0<x<1;

In the link to the post when we find the roots of the quadratic equation and if we know the sign is "<" we can directly right the roots as "root 1" < x < "root 2". The same way in this case also there are 2 roots 0 and 1 so we can directly write it this way. 0<x<1. Why do we have to consider case 1 also.

Rahul


These are just two different approaches.
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Re: Is X between 0 and 1 ? (1) x^2 is less than x (2) x^3 is [#permalink] New post 08 Jun 2012, 21:14
Hi Bunuel, Am I right in construing when I say that x(x-1)<0, which means the roots are 0, 1 and since it is "<" the solution must lie between 0 and 1 and hence, 0<x<1. Please confirm.
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Re: Is X between 0 and 1 ? (1) x^2 is less than x (2) x^3 is [#permalink] New post 09 Jun 2012, 01:56
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Hi Bunuel, Am I right in construing when I say that x(x-1)<0, which means the roots are 0, 1 and since it is "<" the solution must lie between 0 and 1 and hence, 0<x<1. Please confirm.


Yes, that's correct: x(x-1)<0 --> 0<x<1.

Explained here: x2-4x-94661.html#p731476
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Is X between 0 and 1? [#permalink] New post 01 Mar 2013, 03:37
Is X between 0 and 1?
1. x^2 is less than x.
2. x^3 is positive
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Re: Is X between 0 and 1? [#permalink] New post 01 Mar 2013, 04:07
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Is X between 0 and 1?
1. x^2 is less than x.
2. x^3 is positive


From F.S 1, we have x^2<x

or x*(x-1)<0 . This is possible only if they have different signs. Thus, either x<0 AND (x-1)>0[ This is not possible as x can't be more than 1 and yet be negative] or x>0 AND (x-1)<0. This gives us that 0<x<1. Sufficient.

From F.S 2, we know that x^3 >0. Thus, cancelling out x^2 from both sides, we have x>0. Insufficient.

A.
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Re: Is X between 0 and 1? [#permalink] New post 01 Mar 2013, 08:54
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Re: Is X between 0 and 1 ? (1) x^2 is less than x (2) x^3 is [#permalink] New post 26 Mar 2013, 04:08
One of my favorite number property questions. Really good approach and you need to come to inferences fast on this one.
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Re: Is X between 0 and 1 ? (1) x^2 is less than x (2) x^3 is   [#permalink] 26 Mar 2013, 04:08
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