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# What are the unique values of b and c in the equation 4x^2 +

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What are the unique values of b and c in the equation 4x^2 + [#permalink]

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27 Oct 2010, 12:12
00:00

Difficulty:

55% (hard)

Question Stats:

62% (02:08) correct 38% (02:26) wrong based on 21 sessions

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What are the unique values of b and c in the equation 4x^2 + bx + c = 0 if one of the roots of the equation is (-1/2) ?

I. The second root is 1/2
II. The ratio of c and b is 1
[Reveal] Spoiler: OA

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27 Oct 2010, 14:23
zerotoinfinite2006 wrote:
What are the unique values of b and c in the equation 4x^2 + bx + c = 0 if one of the roots of the equation is (-1/2) ?

I. The second root is 1/2
II. The ratio of c and b is 1

What is the source of this question ? The two statements are contradictory which is not possible on a GMAT question :

From I : The roots are +1/2 & -1/2 ... which means b=0 and c=-1 ... Hence b/c=0
From II : b/c=1
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28 Oct 2010, 13:34
zerotoinfinite2006 wrote:
What are the unique values of b and c in the equation 4x^2 + bx + c = 0 if one of the roots of the equation is (-1/2) ?

I. The second root is 1/2
II. The ratio of c and b is 1

Here is my take.

In the question statement, we are given 4x^2 + bx + c = 0 and one of the roots of the equation is (-1/2). This means that the given quadratic formula has to be simplified into one of the following:

(4x + 2)(x + c/2) where b = 2 + 2c
(2x + 1)(2x + c) where b = 2 + 2c
(x + 1/2)(4x + 2c) where b = 2 + 2c

therefore we need either b or c to get the unique value of b and c.

Statement I:

if the other root is equal to 1/2 then:

(1/2 + c/2) = 0 so c = -1
(2(1/2) + c) = 0 so c = -1
(4(1/2) + 2c) = 0 so c = -1

and b = 0 so sufficient

Statement II:

b/c = 1 so b = c.

since we know from the question that b = 2 + 2c,

b = 2 + 2b

so b = -2 which is also equal to c

so sufficient.

So D is my final answer.
Re: DS Unique value   [#permalink] 28 Oct 2010, 13:34
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