Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.
Customized for You
we will pick new questions that match your level based on your Timer History
Track Your Progress
every week, we’ll send you an estimated GMAT score based on your performance
Practice Pays
we will pick new questions that match your level based on your Timer History
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
It appears that you are browsing the GMAT Club forum unregistered!
Signing up is free, quick, and confidential.
Join other 500,000 members and get the full benefits of GMAT Club
Registration gives you:
Tests
Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.
Applicant Stats
View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more
Books/Downloads
Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!
Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:
Re: Everything is Less Than Zero [#permalink]
10 Feb 2011, 02:28
9
This post received KUDOS
Expert's post
14
This post was BOOKMARKED
144144 wrote:
Thanks Bunuel. +1
A question - what is the best way u use to know if the "good" area is above or below?
i mean - what was the best way for u to know that its between -1/2 to 0
i used numbers ex. 1/4 but it consumes time! is there any better technique?
thanks.
Check the link in my previous post. There are beautiful explanations by gurpreetsingh and Karishma.
General idea is as follows:
We have: \((1+2x)*x^3*(1-2x)<0\) --> roots are -1/2, 0, and 1/2 (equate the expressions to zero to get the roots and list them in ascending order), this gives us 4 ranges: \(x<-\frac{1}{2}\), \(-\frac{1}{2}<x<0\), \(0<x<\frac{1}{2}\) and \(x>\frac{1}{2}\) --> now, test some extreme value: for example if \(x\) is very large number than first two terms ((1+2x) and x) will be positive but the third term will be negative which gives the negative product, so when \(x>\frac{1}{2}\) the expression is negative. Now the trick: as in the 4th range expression is negative then in 3rd it'll be positive, in 2nd it'l be negative again and finally in 1st it'll be positive: + - + -. So, the ranges when the expression is negative are: \(-\frac{1}{2}<x<0\) (2nd range) or \(x>\frac{1}{2}\) (4th range).
Re: Everything is Less Than Zero [#permalink]
13 Feb 2011, 21:05
14
This post received KUDOS
Expert's post
7
This post was BOOKMARKED
subhashghosh wrote:
Hi Bunuel
I'm getting D as answer :
x^3(1-2x)(1+2x) < 0
\(-ve --- -1/2---- +ve--- 0----- -ve-----1/2--- +ve\) Could you please explain where I'm wrong ?
Regards, Subhash
Even though your question is directed to Bunuel, I will give a quick explanation.
The concept of the rightmost section being positive is applicable when every term is positive in the rightmost region. This is the case whenever the expressions involved are of the form (x - a) or (ax - b) etc. When you have a term such as (1-2x), the rightmost region becomes negative. So either, as Bunuel mentioned, check for an extreme value of x or convert (1-2x) to (2x - 1) and flip the sign to >. _________________
Re: Everything is Less Than Zero [#permalink]
02 Mar 2011, 17:40
Karishma I flipped the sign before. So I got x^3(2x-1)(2x-1) > 0
2 cases - both +ve or both -ve
case 1 ------- x > 0 and |x| > 1/2. Hence x > 1/2
case 2 ------ x < 0 and 4x^2 - 1 < 0 x < 0 and -1/2 < x < 1/2 Taking the most restrictive value- -1/2 < x < 0
I hope this is correct. Btw this is 750 level in 2 mins.
VeritasPrepKarishma wrote:
subhashghosh wrote:
Hi Bunuel
I'm getting D as answer :
x^3(1-2x)(1+2x) < 0
\(-ve --- -1/2---- +ve--- 0----- -ve-----1/2--- +ve\) Could you please explain where I'm wrong ?
Regards, Subhash
Even though your question is directed to Bunuel, I will give a quick explanation.
The concept of the rightmost section being positive is applicable when every term is positive in the rightmost region. This is the case whenever the expressions involved are of the form (x - a) or (ax - b) etc. When you have a term such as (1-2x), the rightmost region becomes negative. So either, as Bunuel mentioned, check for an extreme value of x or convert (1-2x) to (2x - 1) and flip the sign to >.
Re: Everything is Less Than Zero [#permalink]
20 Jun 2012, 21:42
4
This post received KUDOS
Expert's post
2
This post was BOOKMARKED
gmatpapa wrote:
Which of the following represents the complete range of x over which x^3 - 4x^5 < 0?
(A) 0 < |x| < ½ (B) |x| > ½ (C) –½ < x < 0 or ½ < x (D) x < –½ or 0 < x < ½ (E) x < –½ or x > 0
Responding to a pm: The problem is the same here. How do you solve this inequality: \((1+2x)*x^3*(1-2x)<0\)
Again, there are 2 ways - The long algebraic method: When is \((1+2x)*x^3*(1-2x)\) negative? When only one of the terms is negative or all 3 are negative. There will be too many cases to consider so this is painful.
The number line method: Multiply both sides of \((1+2x)*x^3*(1-2x)<0\) by -1 to get \((2x + 1)*x^3*(2x - 1)>0\) Take out 2 common to get \(2(x + 1/2)*x^3*2(x - 1/2)>0\) [because you want each term to be of the form (x + a) or (x - a)] Now plot them on the number line and get the regions where this inequality holds. Basically, you need to go through this entire post: inequalities-trick-91482.html _________________
Re: Which of the following represents the complete range of x [#permalink]
01 Nov 2012, 15:06
5
This post received KUDOS
Bunuel Thanx a trillion for your post on solving inequalities using graph You know i paid over 300$ to test prep institutes but got nothing out of it.......when i asked such basic question the tutor got frustrated and insulted me.....But hats off to you... MAx wat will i give 1 kudo...... Wat an expeirence it has been with GMAt club
Thanx a lot Bunuel
Trillion kudos to you and Hats off to you for addressing problems with patience..............I cant express myself how satisfied i am feeling.
Re: Which of the following represents the complete range of x [#permalink]
15 Nov 2012, 12:58
Hi All,
Could I conclude that for this case i.e (1+2x)*x^3*(1-2x)<0 even if one of the terms <0, that does not necessarily mean that the entire product of the 3 terms <0. Cause like if the eq was (1+2x)*x^3*(1-2x)= 0 ....I could have safely concluded that However in this case for the entire product <0.. either 1 terms or 2 terms or even all 3 terms can be - ve.
Re: Which of the following represents the complete range of x [#permalink]
15 Nov 2012, 18:03
1
This post received KUDOS
Expert's post
lesnin wrote:
Hi All,
Could I conclude that for this case i.e (1+2x)*x^3*(1-2x)<0 even if one of the terms <0, that does not necessarily mean that the entire product of the 3 terms <0. Cause like if the eq was (1+2x)*x^3*(1-2x)= 0 ....I could have safely concluded that However in this case for the entire product <0.. either 1 terms or 2 terms or even all 3 terms can be - ve.
When you have product of two or more terms, the product will be negative only when odd number of terms are negative i.e. either only one term is negative and rest are positive or only 3 terms are negative and rest are positive or only 5 terms are negative and rest are positive. (-)(+)(+) = (-) (-)(-)(+) = (+) (-)(-)(-) = (-) _________________
Re: Which of the following represents the complete range of x [#permalink]
17 Jun 2013, 10:57
This might be a difficult question to answer, but here it is:
I understand the methodology in how the correct answer was arrived at (thanks, Bunuel) but my question is, how do I know to use that methodology with this particular question?
Also, could I solve for this problem using x^3(1-4x^2)<0 as opposed to (1+2x)*x^3*(1-2x)<0?
As always, thanks to the community for all of your help.
Re: Which of the following represents the complete range of x [#permalink]
02 Jul 2013, 10:17
gmatpapa wrote:
Which of the following represents the complete range of x over which x^3 – 4x^5 < 0?
A. 0 < |x| < ½ B. |x| > ½ C. –½ < x < 0 or ½ < x D. x < –½ or 0 < x < ½ E. x < –½ or x > 0
my take substitute values 1/2 is in many options try plug in and u find only c, is going correctly, as u can know > 1/2 all work but d also contends , when u look at D first part it leave the race
Re: Which of the following represents the complete range of x [#permalink]
09 Jul 2013, 19:15
Which of the following represents the complete range of x over which x^3 – 4x^5 < 0?
x^3 – 4x^5 < 0 x^3(1-4x^2) < 0 (1-4x^2) < 0 1 < 4x^2 √1 < √4x^2 (when you take the square root of 4x^2 you take the square root of a square so...) 1 < |2x|
1<(2x) 1/2 < x OR 1<-2x -1/2>x
I am still a bit confused as to how we get 0. I see how it is done with the "root" method but my way of solving was just a bit different. Any thoughts?
gmatclubot
Re: Which of the following represents the complete range of x
[#permalink]
09 Jul 2013, 19:15
The “3 golden nuggets” of MBA admission process With ten years of experience helping prospective students with MBA admissions and career progression, I will be writing this blog through...
You know what’s worse than getting a ding at one of your dreams schools . Yes its getting that horrid wait-listed email . This limbo is frustrating as hell . Somewhere...