ps 6.7 : GMAT Problem Solving (PS)

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ps 6.7 [#permalink]

05 Jul 2011, 22:20

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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

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Re: ps 6.7 [#permalink]

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

The answer is A
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Re: ps 6.7 [#permalink]

06 Jul 2011, 01:01

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, 01:01

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