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gmatprept1q19

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gmatprept1q19 [#permalink] New post 20 Jun 2009, 17:51
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please solve
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Schools: Haas School of Business
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Re: gmatprept1q19 [#permalink] New post 20 Jun 2009, 19:31
The answer is C

Asking at what two points does the graph of y = (x+a)(x+b) intersect the x-axis?
i.e. when x=? when y=0.

\Rightarrow (x+a)(x+b) = 0

\Rightarrow x(x+b)+a(x+b) = 0

\Rightarrow x^2 + xb+ ax + ab = 0

\Rightarrow x^2 + x(a+b) + ab = 0

We need to know a+b=? and ab=? to answer the question.

Looking at Statement 1

1. a+b=-1

\Rightarrow x^2 + (-1)x + ab = 0

we still don't know ab=?

Hence Insufficient.


2. The graph intersects the x-axis at (0,-6)

i.e when x=0, y=-6

y = (x+a)(x+b)

\Rightarrow -6 = (0+a)(0+b)

\Rightarrow -6 = ab

Plugging in this value into

x^2 + x(a+b) + ab = 0

\Rightarrow x^2 + x(a+b) - 6 = 0

We are now missing the value for (a+b)

Hence Insufficient

Looking at Statement 1 and 2 together gives us

(a+b)=-1 and ab=-6

Plugging into

x^2 + x(a+b) + ab = 0

\Rightarrow x^2 - x - 6 = 0

We can now solve for x. Hence both statements together are sufficient. The answer is C
Re: gmatprept1q19   [#permalink] 20 Jun 2009, 19:31
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