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Gmatprep coordinate geometry


 
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maolivie
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PostPosted: Thu Jul 05, 2007 7:00 pm    Post subject: Gmatprep coordinate geometry Reply with quote
On the X-Y 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)

OA is C (highlight)
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givemeanid
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PostPosted: Thu Jul 05, 2007 7:18 pm    Post subject: Re: Gmatprep coordinate geometry Reply with quote
maolivie wrote:
On the X-Y 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)

OA is C (highlight)



y = (x+a)*(x+b)
The graph intersects the x-axis at y = 0.
So, we need to find solution for (x+a)(x+b) = 0 OR in other words, we need to find a and b.

(1) a+b = -1
One equation, two unknowns.
NOT SUFFICIENT.

(2) y-intercept is (0, -6). Since this is a point on the graph,
-6 = (0+a)(0+b)
ab = -6
One equation, two unknowns.
NOT SUFFICIENT.


Taking both together, we have two equations and two unknowns. SUFFICIENT.
Answer is C.
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maolivie
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PostPosted: Thu Jul 05, 2007 7:49 pm    Post subject: Reply with quote
thanks givemeanid. I never thought on that wavelength for this problem (where if it lies on the x axis, y naturally is 0). That solved a ton of problems.

Thanks again
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