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# If one root of the equation 2x^2 + 3x – k = 0 is 6, what is the value

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If one root of the equation 2x^2 + 3x – k = 0 is 6, what is the value [#permalink]

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17 Apr 2011, 09:17
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If one root of the equation 2x^2 + 3x – k = 0 is 6, what is the value of k?

(A) 90
(B) 42
(C) 18
(D) 10
(E) –10
[Reveal] Spoiler: OA

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petrifiedbutstanding

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Re: If one root of the equation 2x^2 + 3x – k = 0 is 6, what is the value [#permalink]

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17 Apr 2011, 09:26
Because 6 is a root, the following is true :

2* 6^2 + 3*6 - k = 0

=> k = 72 + 18 = 90

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Re: If one root of the equation 2x^2 + 3x – k = 0 is 6, what is the value [#permalink]

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17 Apr 2011, 13:33
nice,I used the roots equation
=> 6 = (-3 + sqrt(9+8K))/2*2
=> k = 90
So, A
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06 Jul 2011, 00:47
We just enter this root into the equation in order to recieve an equation to find the answer!

2*6^2+3*6-k=0
k=72+18=90

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06 Jul 2011, 01:01
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racejockey wrote:
If one root of the equation 2x2 + 3x – k = 0 is 6, what is the value of k?
(A) 90
(B) 42
(C) 18
(D) 10
(E) –10

The question should say that k is a constant. In that case, if 6 is a 'root' of the equation, that just means that x=6 is one solution to the equation. So we can plug in x=6:
\begin{align*} 2x^2 + 3x - k &= 0 \\ 2(6^2) + (3)(6) - k &= 0 \\ 72 + 18 &= k \\ 90 &= k \end{align*}
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Re: If one root of the equation 2x^2 + 3x – k = 0 is 6, what is the value [#permalink]

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22 Aug 2015, 04:40
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Re: If one root of the equation 2x^2 + 3x – k = 0 is 6, what is the value [#permalink]

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22 Aug 2015, 05:28
petrifiedbutstanding wrote:
If one root of the equation 2x^2 + 3x – k = 0 is 6, what is the value of k?

(A) 90
(B) 42
(C) 18
(D) 10
(E) –10

let the other root is 'a'
a +6=-3/2; a = -15/2
6a = -k/2
implies k=90

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Re: If one root of the equation 2x^2 + 3x – k = 0 is 6, what is the value   [#permalink] 22 Aug 2015, 05:28
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