DS Question from GMAT Prep

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mpaudena: Senior | Next Rank: 100 Posts; Posts: 80; Joined: Wed Jun 04, 2008 1:16 pm; Thanked: 3 times

by mpaudena » Fri Dec 26, 2008 2:55 pm

Hello Gals and Guys. I downloaded the GMAT Prep on Vista and its not allowing me to see the explanation to the CAT answer. I have a question, I'd be greatful to the answerer:

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)

Anyone? Thanks in advance.

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Source: Beat The GMAT — Data Sufficiency | Fri Dec 26, 2008 2:55 pm

crazykrans: Junior | Next Rank: 30 Posts; Posts: 15; Joined: Mon Dec 08, 2008 6:53 am; Thanked: 1 times

by crazykrans » Fri Dec 26, 2008 3:06 pm

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)

At the intersection of X_axis, y=0 -> (x+a)(x+b) = 0 x=-a or -b i.e, the line intersects in the 2nd quadrant

1. a+b = -1 possible but we don't know about a and b. INSUFF

2. ab = -6 not possible by itself. INSUFF

1 and 2, a-6/a = -1 -> a^2+a-6=0 -> a=3 or -2 also, b=-2 or3

the 2 points are (-3,0) and (-2,0).

Ans: C

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mpaudena: Senior | Next Rank: 100 Posts; Posts: 80; Joined: Wed Jun 04, 2008 1:16 pm; Thanked: 3 times

by mpaudena » Fri Dec 26, 2008 3:28 pm

crazykrans wrote: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)

At the intersection of X_axis, y=0 -> (x+a)(x+b) = 0 x=-a or -b i.e, the line intersects in the 2nd quadrant

1. a+b = -1 possible but we don't know about a and b. INSUFF

2. ab = -6 not possible by itself. INSUFF

1 and 2, a-6/a = -1 -> a^2+a-6=0 -> a=3 or -2 also, b=-2 or3

the 2 points are (-3,0) and (-2,0).

Ans: C

Okay, here is where you lose me: "2. ab = 6" How did you get ab = -6?

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crazykrans: Junior | Next Rank: 30 Posts; Posts: 15; Joined: Mon Dec 08, 2008 6:53 am; Thanked: 1 times

by crazykrans » Fri Dec 26, 2008 3:33 pm

y=(x+a)(x+b) passes through (0,-6) -> -6=(0+a)(0+b) ->ab = -6

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mpaudena: Senior | Next Rank: 100 Posts; Posts: 80; Joined: Wed Jun 04, 2008 1:16 pm; Thanked: 3 times

by mpaudena » Fri Dec 26, 2008 3:42 pm

Thanks. Got it.

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