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xy plane

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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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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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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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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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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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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