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Which of the following describes all the values of x for whi

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New post 30 Jul 2014, 01:53
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Which of the following describes all the values of x for which x(x−1)(x−2)≥0?

A. 0 ≤ x ≤ 1 or x ≥ 2
B. x ≤ 0 or x ≥ 2
C. 0 ≤ x ≤ 1
D. 1 ≤ x ≤ 2 or x ≥ 2
E. 0 ≤ x ≤ 2
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Which of the following describes all the values of x for whi  [#permalink]

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New post 30 Jul 2014, 02:18
2
goodyear2013 wrote:
Which of the following describes all the values of x for which x(x−1)(x−2)≥0?

A. 0 ≤ x ≤ 1 or x ≥ 2
B. x ≤ 0 or x ≥ 2
C. 0 ≤ x ≤ 1
D. 1 ≤ x ≤ 2 or x ≥ 2
E. 0 ≤ x ≤ 2


In terms of graphs, Cubic equation has 3 roots : x=0, 1 , 2.
for, X >=2 all three terms positive => graph will hit x axis at x=2 from downward to upward and keep on going up as x increases from 2.
=> x<2 graph is in negative zone : till when ? => till next root point i.e. x=1 ..
And so on so forth ..
Voila It's A. Graphs made it really easy !
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Re: Which of the following describes all the values of x for whi  [#permalink]

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New post 30 Jul 2014, 02:40
1
3
goodyear2013 wrote:
Which of the following describes all the values of x for which x(x−1)(x−2)≥0?

A. 0 ≤ x ≤ 1 or x ≥ 2
B. x ≤ 0 or x ≥ 2
C. 0 ≤ x ≤ 1
D. 1 ≤ x ≤ 2 or x ≥ 2
E. 0 ≤ x ≤ 2


x(x−1)(x−2)≥0 --> the "roots" are 0, 1 and 2, this gives us 4 ranges:

x<0;
0≤x≤1;
1<x<2;
x≥2.

Next, test some extreme value for x: if x is some large enough number, say 10, then all three multiples will be positive which gives the positive result for the whole expression, so when x≥2, the expression is positive. Now the trick: as in the 4th range expression is positive, then in the 3rd it'll be negative, in the 2nd it'll ne positive and finally in the 1st it'll be negative: - + - + . So, the ranges when the expression is positive or 0 are: 0≤x≤1 and x≥2.

Answer: A.


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Re: Which of the following describes all the values of x for whi  [#permalink]

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New post 04 Aug 2014, 23:40
I'm not getting this - could anybody please elaborate a bit more.

Is there any way els around this than presented by Bunuel? Thx
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Re: Which of the following describes all the values of x for whi  [#permalink]

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New post 08 Jul 2015, 06:34
Bunuel wrote:
goodyear2013 wrote:
Which of the following describes all the values of x for which x(x−1)(x−2)≥0?

A. 0 ≤ x ≤ 1 or x ≥ 2
B. x ≤ 0 or x ≥ 2
C. 0 ≤ x ≤ 1
D. 1 ≤ x ≤ 2 or x ≥ 2
E. 0 ≤ x ≤ 2


x(x−1)(x−2)≥0 --> the "roots" are 0, 1 and 2, this gives us 4 ranges:

x<0;
0≤x≤1;
1<x<2;
x≥2.

Next, test some extreme value for x: if x is some large enough number, say 10, then all three multiples will be positive which gives the positive result for the whole expression, so when x≥2, the expression is positive. Now the trick: as in the 4th range expression is positive, then in the 3rd it'll be negative, in the 2nd it'll ne positive and finally in the 1st it'll be negative: - + - + . So, the ranges when the expression is positive or 0 are: 0≤x≤1 and x≥2.

Answer: A.



Hi Bunuel,

I am able to understand 99% of it and got the answer right, but I have one question.

Why <= is used for some sets of roots only.

x<0;
0≤x≤1;
1<x<2;
x≥2.
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Re: Which of the following describes all the values of x for whi  [#permalink]

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New post 08 Jul 2015, 07:52
robinpallickal wrote:
Bunuel wrote:
goodyear2013 wrote:
Which of the following describes all the values of x for which x(x−1)(x−2)≥0?

A. 0 ≤ x ≤ 1 or x ≥ 2
B. x ≤ 0 or x ≥ 2
C. 0 ≤ x ≤ 1
D. 1 ≤ x ≤ 2 or x ≥ 2
E. 0 ≤ x ≤ 2


x(x−1)(x−2)≥0 --> the "roots" are 0, 1 and 2, this gives us 4 ranges:

x<0;
0≤x≤1;
1<x<2;
x≥2.

Next, test some extreme value for x: if x is some large enough number, say 10, then all three multiples will be positive which gives the positive result for the whole expression, so when x≥2, the expression is positive. Now the trick: as in the 4th range expression is positive, then in the 3rd it'll be negative, in the 2nd it'll ne positive and finally in the 1st it'll be negative: - + - + . So, the ranges when the expression is positive or 0 are: 0≤x≤1 and x≥2.

Answer: A.



Hi Bunuel,

I am able to understand 99% of it and got the answer right, but I have one question.

Why <= is used for some sets of roots only.

x<0;
0≤x≤1;
1<x<2;
x≥2.


The question asks for the ranges of x for which x(x−1)(x−2)0. So, the transition points, "roots" (0, 1, and 2) must be included in ranges we consider (x<0; 0≤x≤1; 1<x<2; x≥2.). Actually it does not matter to which number you apply = sign as long as you apply it to all of them (0, 1, and 2).
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
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Which of the following describes all the values of x for whi  [#permalink]

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New post 09 Jul 2015, 02:17
Thanks a lot Bunuel.
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Re: Which of the following describes all the values of x for whi  [#permalink]

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New post 31 Oct 2018, 11:24
Bunuel wrote:
goodyear2013 wrote:
Which of the following describes all the values of x for which x(x−1)(x−2)≥0?

A. 0 ≤ x ≤ 1 or x ≥ 2
B. x ≤ 0 or x ≥ 2
C. 0 ≤ x ≤ 1
D. 1 ≤ x ≤ 2 or x ≥ 2
E. 0 ≤ x ≤ 2


x(x−1)(x−2)≥0 --> the "roots" are 0, 1 and 2, this gives us 4 ranges:

x<0;
0≤x≤1;
1<x<2;
x≥2.

Next, test some extreme value for x: if x is some large enough number, say 10, then all three multiples will be positive which gives the positive result for the whole expression, so when x≥2, the expression is positive. Now the trick: as in the 4th range expression is positive, then in the 3rd it'll be negative, in the 2nd it'll ne positive and finally in the 1st it'll be negative: - + - + . So, the ranges when the expression is positive or 0 are: 0≤x≤1 and x≥2.

Answer: A.



Hi Bunuel,

The graph inequalities strategy appears good for 2 solutions, can you kindly explain how you are assigning signs here and if the approach is always applicable.
Thanks in advance for answering.
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Re: Which of the following describes all the values of x for whi &nbs [#permalink] 31 Oct 2018, 11:24
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Which of the following describes all the values of x for whi

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