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# ps 6.7

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ps 6.7 [#permalink]  05 Jul 2011, 21:20
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100% (01:28) correct 0% (00:00) wrong based on 4 sessions
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
[Reveal] Spoiler: OA
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Joined: 03 Mar 2011
Posts: 90
Location: United States
Schools: Erasmus (S)
GMAT 1: 730 Q51 V37
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Re: ps 6.7 [#permalink]  05 Jul 2011, 23:47
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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Re: ps 6.7 [#permalink]  06 Jul 2011, 00:01
Expert's post
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: ps 6.7   [#permalink] 06 Jul 2011, 00:01
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