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# If x > 0, the value of -2√(5x) + x + 9 is least when x

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Current Student
Joined: 27 May 2014
Posts: 524
GMAT 1: 730 Q49 V41
If x > 0, the value of -2√(5x) + x + 9 is least when x  [#permalink]

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04 Mar 2018, 23:04
1
1
00:00

Difficulty:

35% (medium)

Question Stats:

74% (00:46) correct 26% (01:25) wrong based on 31 sessions

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If x > 0, the value of -2√(5x) + x + 9 is least when x = ?

A) 0
B) 1
C) √5
D) 4
E) 5

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Joined: 07 Dec 2017
Posts: 868
Re: If x > 0, the value of -2√(5x) + x + 9 is least when x  [#permalink]

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05 Mar 2018, 01:34
saswata4s wrote:
If x > 0, the value of -2√(5x) + x + 9 is least when x = ?

A) 0
B) 1
C) √5
D) 4
E) 5

Since it's hard to see where to start, we'll estimate the answer.
This is an Alternative approach.

We'll estimate the values of the square root, rounding it to the nearest integer (for example √5 = about 2)

(A) gives 0 + 0 +9 = 9
(B) gives -2√5 + 10 = about -4 + 10 = 6
(C) gives -2√(5√5) +√5 + 9 = about -2√10 + 11 = about -6 + 11 = 5
(D) gives -2√20 + 4 + 9 = about -8 + 4 + 9 = 5
(E) gives -2√25 + 5 + 9 = -10 + 5 + 9 = 4

Then (E) is our answer.
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Joined: 31 Jul 2017
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Schools: INSEAD Jan '19
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If x > 0, the value of -2√(5x) + x + 9 is least when x  [#permalink]

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06 Mar 2018, 03:50
saswata4s wrote:
If x > 0, the value of -2√(5x) + x + 9 is least when x = ?

A) 0
B) 1
C) √5
D) 4
E) 5

We can do it by Substituting... Though there is a Simpler Approach for this using Differentiation.

Let Min = $$-2√(5x) + x + 9$$
By Differentiating, we get -

$$1 - 2(\frac{1}{2})\frac{√5}{√x}$$ = 0
x = 5

Bunuel - You may please remove this post if its a violation of the policy

--== Message from the GMAT Club Team ==--

THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION.
This discussion does not meet community quality standards. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

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If x > 0, the value of -2√(5x) + x + 9 is least when x &nbs [#permalink] 06 Mar 2018, 03:50
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# If x > 0, the value of -2√(5x) + x + 9 is least when x

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