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DS problem - graph

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gmatme: Junior | Next Rank: 30 Posts; Posts: 18; Joined: Tue Mar 06, 2007 8:57 am

by gmatme » Sun Mar 18, 2007 8:24 am

How the first statement is sufficient?

In the xy-plane, at what two points does the graph of y = (x+a)(x+b) intersect the x-axis?

1) a+b = -1
2) The graph intersects the y-axis at (0,-6)

Ans is C ?

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Source: Beat The GMAT — Data Sufficiency | Sun Mar 18, 2007 8:24 am

jayhawk2001: Community Manager; Posts: 789; Joined: Sun Jan 28, 2007 3:51 pmFollowed by: 1 member; Location: Silicon valley, California; Thanked: 30 times

by jayhawk2001 » Sun Mar 18, 2007 8:39 am

1 - insufficient. All we have it x^2 -x + ab = 0. Cannot
solve for x

2 - insufficient. y = x^2 + (a+b)x + ab implies ab = -6. Cannot
solve for x

Using 1 and 2

x^2 - x - 6 = 0
x = 3 & -2

So, C looks like the correct answer

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gmatme: Junior | Next Rank: 30 Posts; Posts: 18; Joined: Tue Mar 06, 2007 8:57 am

by gmatme » Sun Mar 18, 2007 8:56 am

Can you please explain why ab = -6?

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gmatme: Junior | Next Rank: 30 Posts; Posts: 18; Joined: Tue Mar 06, 2007 8:57 am

by gmatme » Sun Mar 18, 2007 9:23 am

I know why ab=-6, please ignore my last post.

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