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

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

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New post 05 Mar 2018, 00:04
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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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Re: If x > 0, the value of -2√(5x) + x + 9 is least when x  [#permalink]

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New post 05 Mar 2018, 02:34
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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If x > 0, the value of -2√(5x) + x + 9 is least when x  [#permalink]

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New post 06 Mar 2018, 04:50
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

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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, 04:50
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