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# For what value of x between -5 and 1 inclusive, the value of 6x^2-x^4

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Director
Joined: 18 Feb 2019
Posts: 581
Location: India
GMAT 1: 460 Q42 V13
GPA: 3.6
For what value of x between -5 and 1 inclusive, the value of 6x^2-x^4  [#permalink]

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14 Mar 2019, 09:40
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Difficulty:

65% (hard)

Question Stats:

50% (01:49) correct 50% (01:52) wrong based on 82 sessions

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For what value of x between -5 and 1 inclusive, the value of 6x^2–x^4-4 is greatest?

A. -5
B. -√5
C. -2
D. -√3
E. 1
Manager
Joined: 21 Feb 2019
Posts: 125
Location: Italy
Re: For what value of x between -5 and 1 inclusive, the value of 6x^2-x^4  [#permalink]

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14 Mar 2019, 09:56
2
I differentiated:

$$12x - 4x^3 = 0$$

$$x^3 - 3x = 0$$

$$x(x^2 - 3) = 0$$

$$x = {0, +\sqrt{3}, -\sqrt{3}}$$

Since there is only $$-\sqrt{3}$$ among the options, pick it up.

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MEMENTO AUDERE SEMPER
Manager
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Location: India
Concentration: Finance, Entrepreneurship
GMAT 1: 710 Q49 V38
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Re: For what value of x between -5 and 1 inclusive, the value of 6x^2-x^4  [#permalink]

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14 Mar 2019, 12:55
kiran120680 wrote:
For what value of x between -5 and 1 inclusive, the value of 6x^2–x^4-4 is greatest?

A. -5
B. -√5
C. -2
D. -√3
E. 1

if we use -5,it will give a very large negative value, so cancel it. the larger the value,the more negative it will keep getting. just check for -\sqrt{3} and 1.

Ans:D
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GMAT:710 (Q49,V38,IR8,AWA5)
Director
Joined: 18 Feb 2019
Posts: 581
Location: India
GMAT 1: 460 Q42 V13
GPA: 3.6
Re: For what value of x between -5 and 1 inclusive, the value of 6x^2-x^4  [#permalink]

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15 Mar 2019, 07:12
lucajava wrote:
I differentiated:

$$12x - 4x^3 = 0$$

$$x^3 - 3x = 0$$

$$x(x^2 - 3) = 0$$

$$x = {0, +\sqrt{3}, -\sqrt{3}}$$

Since there is only $$-\sqrt{3}$$ among the options, pick it up.

Manager
Joined: 21 Feb 2019
Posts: 125
Location: Italy
Re: For what value of x between -5 and 1 inclusive, the value of 6x^2-x^4  [#permalink]

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15 Mar 2019, 10:32
1
kiran120680 wrote:
lucajava wrote:
I differentiated:

$$12x - 4x^3 = 0$$

$$x^3 - 3x = 0$$

$$x(x^2 - 3) = 0$$

$$x = {0, +\sqrt{3}, -\sqrt{3}}$$

Since there is only $$-\sqrt{3}$$ among the options, pick it up.

In math, the first derivative is used to find max. and min. values of a function. Starting from this point, i differentiated to find the $$x$$ where the function takes the greatest value. Hope it's clear.
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MEMENTO AUDERE SEMPER
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Concentration: General Management, Finance
GMAT 1: 730 Q47 V44
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Re: For what value of x between -5 and 1 inclusive, the value of 6x^2-x^4  [#permalink]

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13 May 2019, 10:33
Can someone provide the actual algebraic way to this question?
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Re: For what value of x between -5 and 1 inclusive, the value of 6x^2-x^4   [#permalink] 13 May 2019, 10:33
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