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In the xy-plane, at what two points does the graph of

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In the xy-plane, at what two points does the graph of [#permalink] New post 06 Sep 2008, 19:54
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3. In the xy-plane, at what two points does the graph of y=(x+a)(x+b) intersect the x-axis?
a. A+b = -1
b. The graph intersects the y-axis at (0,-6)
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Re: DS- graph problem [#permalink] New post 06 Sep 2008, 21:50
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ttwang56 wrote:
3. In the xy-plane, at what two points does the graph of y=(x+a)(x+b) intersect the x-axis?
a. A+b = -1
b. The graph intersects the y-axis at (0,-6)


y = x^2 + (a+b)x + ab
Need to know values for a and b

(1) Insufficient
y = x^2 - x + ab --> ab = ?

(2) Insufficient
-6 = 0^2 + 0*(a+b) + ab
ab = -6

(1) and (2) Sufficient
a+b = -1
ab = -6

a = -3
b = 2
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Re: DS- graph problem [#permalink] New post 07 Sep 2008, 09:54
is it reqd to know abt parabolas and their formulas for the GMAT?
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Re: DS- graph problem [#permalink] New post 07 Sep 2008, 10:34
arjtryarjtry wrote:
is it reqd to know abt parabolas and their formulas for the GMAT?


it isn't, but you don't need to know that the equation describes a parabola to be able to solve the problem
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Re: DS- graph problem [#permalink] New post 19 Sep 2008, 13:06
C ... intercepts happen at -a and -b, and thats what we need to find.

from stat 1, we cant determine individual values, since any number of combinations would satisfy the inequality

from stat 2, we know that ab=6 (y intercept happens when x=0, so y=6=(0-a)(0-b))

Together, we have two equations and two unknowns, so we can solve
Re: DS- graph problem   [#permalink] 19 Sep 2008, 13:06
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