xy plane

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shibal: Master | Next Rank: 500 Posts; Posts: 345; Joined: Wed Mar 18, 2009 6:53 pm; Location: Sao Paulo-Brazil; Thanked: 12 times; GMAT Score:660

by shibal » Tue Jun 30, 2009 6:13 pm

In the xy plane, at what point does y = (x + a)(x + b) cross the x axis?
a. a + b = -1
b. graph intersects y axis at (0, -6)
oa c

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Source: Beat The GMAT — Data Sufficiency | Tue Jun 30, 2009 6:13 pm

aj5105: Legendary Member; Posts: 1169; Joined: Sun Jul 06, 2008 2:34 amFollowed by: 1 member; Thanked: 25 times

by aj5105 » Tue Jun 30, 2009 7:35 pm

From 1, a+b=-1.

From 2, x=0, so ab=6.

Combine,

(x+a)*(x+b)=0

x^2+(a+b)x+ab=0

So, x=-3, x=2

The answer is (C)

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LB20: Newbie | Next Rank: 10 Posts; Posts: 9; Joined: Mon Apr 13, 2009 11:33 am; GMAT Score:760

by LB20 » Wed Jul 01, 2009 12:11 pm

If x has two possible answers when you combine statements 1 & 2, how is the answer C? i think since the equation is a parabola, it should cross it 2x, so i'm wondering if the question actually asks what pointS (plural) it crosses the x axis?

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shibal: Master | Next Rank: 500 Posts; Posts: 345; Joined: Wed Mar 18, 2009 6:53 pm; Location: Sao Paulo-Brazil; Thanked: 12 times; GMAT Score:660

by shibal » Wed Jul 01, 2009 4:21 pm

i didn't get it... how you got to the answer -3 and 2?

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rahulg83: Legendary Member; Posts: 575; Joined: Tue Nov 04, 2008 2:58 amFollowed by: 4 members; Location: India; Thanked: 18 times; GMAT Score:710

Re: xy plane

by rahulg83 » Wed Jul 01, 2009 9:13 pm

shibal wrote:In the xy plane, at what point does y = (x + a)(x + b) cross the x axis?
a. a + b = -1
b. graph intersects y axis at (0, -6)
oa c

St 1: a+b=-1
y=x^2+(a+b)x+ab=y=x^2-x+ab...Insuff
St 2: put y=-6, x=0 => ab=-6...Insuff

Combining both, we get the equation y=x^2-x-6
this is a parabola intersecting x axis at (x,y)=(-3,2)...therefore C..

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yogami: Master | Next Rank: 500 Posts; Posts: 170; Joined: Tue May 26, 2009 12:00 pm; Thanked: 5 times

by yogami » Thu Jul 02, 2009 12:05 am

I think the question makes sense if it is reworded as "at what points does it cross the x axis" instead of "at what point"
there are two points 3 and -2

200 or 800. It don't matter no more.

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