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In the xy-plane at what points does the graph of y

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Director
Joined: 10 Feb 2006
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In the xy-plane at what points does the graph of y [#permalink]  25 Nov 2007, 19:23
In the xy-plane at what points does the graph of y =(x+a)(x+b) intersect the x-axis?

(1). a + b = -1
(2). The graph intersect the yaxis ast (0,-6)

Solution :

Wait for you solution. thanks
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CEO
Joined: 17 Nov 2007
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Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
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[#permalink]  25 Nov 2007, 22:20
Expert's post
C

y =(x+a)(x+b) - parabola that intersect x-axis at points (-a,0) and (-b,0)
Therefore, we should known both a and b.

1. a + b = -1 (-3,2) and (-2,1) match equation - insufficient
2. The graph intersect the yaxis ast (0,-6) means -6=(0+a)(0+b) ==> ab=-6
(-3,2) and (1,-6) match equation - insufficient

1&2. a + b = -1 and ab=-6 ==> solution: -3 and 2 sufficient
Director
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Re: GMATPREP - [#permalink]  25 Nov 2007, 23:08
also for C,

1) INSUFF

a+b=-1-------------> (a,b) could be 0,-1 ; -5,4 ; or vice versa

2) INSUFF

ab=-6--------------> (a,b) could be 1,-6 ; -2;3 ; or vice versa

both) SUFF

a= -1-b

ab= (-1-b)b
b^2-b-6=0

b=2 or b=-3

b=2 a= -3
b=-3 a=2

in any case parabola intersects x axis at (3,0) and (-2,0)
SVP
Joined: 28 Dec 2005
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[#permalink]  16 Dec 2007, 13:03

statement 1 and 2 give multiple options, not sufficient.

With both, you can nail down that a=-2 and b=3
[#permalink] 16 Dec 2007, 13:03
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