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

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Intern
Joined: 26 Jan 2008
Posts: 25
In the xy-plane, at what two points does the graph of  [#permalink]

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06 Sep 2008, 20:54
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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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Director
Joined: 12 Jul 2008
Posts: 511
Schools: Wharton

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06 Sep 2008, 22:50
1
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
Current Student
Joined: 11 May 2008
Posts: 551

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07 Sep 2008, 10:54
is it reqd to know abt parabolas and their formulas for the GMAT?
Director
Joined: 12 Jul 2008
Posts: 511
Schools: Wharton

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07 Sep 2008, 11:34
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
VP
Joined: 28 Dec 2005
Posts: 1477

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19 Sep 2008, 14:06
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

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This is not a quality discussion. It has been retired.

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Re: DS- graph problem &nbs [#permalink] 19 Sep 2008, 14:06
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