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Re: If |x|<x^2 , which of the following must be true? [#permalink]
04 Oct 2012, 20:21

1

This post received KUDOS

Expert's post

idreesma wrote:

Hi Krishma, the link that you posted is probably best for summary. It ends with the comment "Anyone wants to add ABSOLUTE VALUES....That will be a value add to this post ". Can you suggest any such thread where absolute value side is summarized like this as well. Appreciate your input Thanks

(A) I only (B) II only (C) III only (D) I and II only (E) I and III only

what is the fastest way to solbe it ?

Given: |x|<x^2 --> reduce by |x| (side note: we can safely do this as absolute value is non-negative and in this case we know it's not zero too, as if x would be zero inequality wouldn't hold true, so |x|>0) --> 1<|x| --> x<-1 or x>1.

So we have that x<-1 or x>1.

I. x^2>1 --> always true; II. x>0 --> may or may not be true; III. x<-1 --> --> may or may not be true.

Answer: A (I only).

Hi bunuel, sorry , but I didn't undestrand how you reduce |x|<x^2 by |x| to get the solution...x<-1 and x>1 please, could you explain me in more details? Thanks

(A) I only (B) II only (C) III only (D) I and II only (E) I and III only

what is the fastest way to solbe it ?

Given: |x|<x^2 --> reduce by |x| (side note: we can safely do this as absolute value is non-negative and in this case we know it's not zero too, as if x would be zero inequality wouldn't hold true, so |x|>0) --> 1<|x| --> x<-1 or x>1.

So we have that x<-1 or x>1.

I. x^2>1 --> always true; II. x>0 --> may or may not be true; III. x<-1 --> --> may or may not be true.

Answer: A (I only).

Hi bunuel, sorry , but I didn't undestrand how you reduce |x|<x^2 by |x| to get the solution...x<-1 and x>1 please, could you explain me in more details? Thanks

Express x^2 as |x|*|x|, so we have that: |x|<|x|*|x| --> reduce by |x| (notice that |x| is positive, thus we can safely divide both parts of the inequality by it) --> 1<|x| --> x<-1 or x>1.

Re: If |x|<x^2 , which of the following must be true? [#permalink]
09 Nov 2012, 08:34

I just cannot get the answer, Bunuel can you give a more detailed explanation about how you conjugated the problem. When I solve the equation in the question I get this --> |x| < x^2 -->

-|x| < x^2 --> -x < x^2 --> divide by x --> -1 < x |x| < x^2 --> x < x^2 --> divide by x --> 1 < x

which makes the first equation redundant. How do you get x < -1 ?????

Re: If |x|<x^2 , which of the following must be true? [#permalink]
05 Dec 2012, 03:04

|x|<x^2

Determine thw value of x first before testing agains I,II and III. Let us find out x by testing against checkpoints -1, 0 and 1. Let x < -1: |-2| < 4 Yes! Let x =-1: 1 < 1 No! Let -1 < x < 0: x=-1/4 : 1/4 < 1/16 No! Let 0 < x < 1 : x =1/4 : 1/4 < 1/16 No! Let x = 1: 1 < 1 No! Let x > 1: x = 2 : 2 < 4 Yes!

Answer: -1 < x < 1 or |x| > 1

I. x^2>1 Remember, x^2 > 1 is the same as |x| > 1 Yes! II. x>0 This doesn't cover x < -1. III. x<-1 This doesn't cover x > 1.

Bunuel, Isn't this always true when |x|<x^2 ... Isn't the answer E.

Thanks.

|x|<x^2 is given as fact and then we asked to determine which of the following statements MUST be true.

|x|<x^2 means that either x<-1 or x>1, x can be ANY value from these two ranges, (I think in your own solution you've reached this conclusion: when x<-1 the graph of |x| is below (less than) the graph of x^2 and when x>1 again the graph of |x| is below the graph of x^2).

Now, III says x<-1 this statement is not always true as x can be for example 3 and in this case x<-1 doesn't hold true.

Re: If |x|<x^2 , which of the following must be true? [#permalink]
07 Mar 2013, 09:50

Here are my two cents.

You cannot divide both the sides by only x since you do not know whether x is positive or not. Granted that the absolute value will always be positive but if x=-1, absolute value would be 1 which would be denoted as -x. Hence, the absolute value, even though positive, could be +/-x.

Now take the negative aspect into the picture.

|x|<x^2 -x<x^2 -1>x Hence, for values such as x <-1, the equation stands. You can test it with values like x=-2,-3 etc.

Hence, c does not stand as it does not necessarily have to be true. Of course, that was evident from you calculations.

Hope this helps!

BDSunDevil wrote:

Bunuel: I have solved the problem in the following manner: Absolute value of x is positive. So, we can divide both side by x. that leaves us, 1<x or x>1. or x^2>1 Though i have the OA, am i making any fundamental error?

Re: If |x|<x^2 , which of the following must be true? [#permalink]
09 Mar 2013, 22:24

mehdiov wrote:

If |x|<x^2 , which of the following must be true?

I. x^2>1 II. x>0 III. x<-1

(A) I only (B) II only (C) III only (D) I and II only (E) I and III only

PLEASE READ THE WHOLE THREAD AND FOLLOW THE LINKS PROVIDED IN ORDER TO UNDERSTAND THE QUESTION/SOLUTION CORRECTLY.

|x| - x^2 < 0

If x>0, x ( 1 - x) < 0--> x < 0 (can't be true as condition states x>0or x>1 If x<0, -x - x^2 <0-----> x(1+x) >0 --> x>0 or x> -1.

x>1 & x>-1 only option A is valid. x can't be equal to 0. as 0>0 is not true. Is my solution correct?? Why it is different then Bunuel's solution
_________________

|x| will be less than x^2 in the region where the graph of |x| is below the graph of x^2. Graph of y = |x| is a V at (0, 0) and graph of y = x^2 is an upward facing parabola at (0, 0)

Attachment:

Ques3.jpg [ 7.72 KiB | Viewed 325 times ]

Just looking at the graph should tell you that answer must be (A). You don't need to solve. x will lie in one of two ranges x > a or x < -a. II and III will not work in any case. I must work since one of them has to be true according to the options.
_________________

Re: If |x|<x^2 , which of the following must be true? [#permalink]
01 Jun 2013, 09:08

The trick is in question, it doesn't ask you to choose possible answers, but it asks for an answer which holds true in all possibilities. Option (i) satisfies this always, but option (iii), not always..
_________________

And many strokes, though with a little axe, hew down and fell the hardest-timbered oak. - William Shakespeare

Re: If |x|<x^2 , which of the following must be true? [#permalink]
09 Jul 2013, 21:52

If |x|<x^2 , which of the following must be true?

|x|<x^2

I. x^2>1 If |x|<x^2 then that means x^2 is greater than positive value x. x cannot be between -1 and 1 inclusive because then |x| would be greater than or equal to x^2, not less than it as the stem stipulates. Therefore, for the stem to be true x^2 must be greater than 1.

II. x>0 If x>0 then x could = 1/2. |1/2|<(1/2)^2 1/2<1/4 which is invalid

III. x<-1 While any x value less than -1 will provide a correct answer, valid values for x don't have to be less than -1. They can be greater than one as well.

Re: If |x|<x^2 , which of the following must be true? [#permalink]
21 Jul 2013, 17:47

mehdiov wrote:

If |x|<x^2 , which of the following must be true?

I. x^2>1 II. x>0 III. x<-1

(A) I only (B) II only (C) III only (D) I and II only (E) I and III only

PLEASE READ THE WHOLE THREAD AND FOLLOW THE LINKS PROVIDED IN ORDER TO UNDERSTAND THE QUESTION/SOLUTION CORRECTLY.

Having seen such a big red quote here, I was wary of this question before starting. For questions like these I have created a method which has helped me and would like to share it.

Bunnel, Please correct me if this is wrong or has limited scope.

If |x|<x^2 , which of the following must be true? this means x>1 and x<-1.

The question stem set must be equal to or a subset of the answer choices i.e. The answer choice must contain at-least all of points of the question

I - x^2>1 => x>1 and x<-1 this choice consists of exactly the same data points as in the question stem, subset and hence, Yes.

II - X>0 this choice does not contain all of the points of the question stem, not a subset and hence, No.

III - x<-1 this choice does not contain all of the points of the question stem, not a subset and hence, No.

Try similar question: If |x| > 3, which of the following must be true?

1) x > 3 2) x^2 > 9 3) |x - 1| > 2

I only II only I and II only II and III only I, II, and III

Re: If |x|<x^2 , which of the following must be true? [#permalink]
21 Jul 2013, 20:51

Expert's post

cumulonimbus wrote:

mehdiov wrote:

If |x|<x^2 , which of the following must be true?

I. x^2>1 II. x>0 III. x<-1

(A) I only (B) II only (C) III only (D) I and II only (E) I and III only

PLEASE READ THE WHOLE THREAD AND FOLLOW THE LINKS PROVIDED IN ORDER TO UNDERSTAND THE QUESTION/SOLUTION CORRECTLY.

Having seen such a big red quote here, I was wary of this question before starting. For questions like these I have created a method which has helped me and would like to share it.

Bunnel, Please correct me if this is wrong or has limited scope.

If |x|<x^2 , which of the following must be true? this means x>1 and x<-1.

The question stem set must be equal to or a subset of the answer choices i.e. The answer choice must contain at-least all of points of the question

I - x^2>1 => x>1 and x<-1 this choice consists of exactly the same data points as in the question stem, subset and hence, Yes.

II - X>0 this choice does not contain all of the points of the question stem, not a subset and hence, No.

III - x<-1 this choice does not contain all of the points of the question stem, not a subset and hence, No.

Try similar question: If |x| > 3, which of the following must be true?

1) x > 3 2) x^2 > 9 3) |x - 1| > 2

I only II only I and II only II and III only I, II, and III

Re: If |x|<x^2 , which of the following must be true? [#permalink]
21 Jan 2014, 22:49

This is very tricky. If this is a 600-700 level, I am in trouble. I did get it, however, I don't think I would get it if didn't read the explanation for several times. And still a little confused. Thank you.

gmatclubot

Re: If |x|<x^2 , which of the following must be true?
[#permalink]
21 Jan 2014, 22:49