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Gmatprep coordinate geometry

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Re: Gmatprep coordinate geometry

by givemeanid » Thu Jul 05, 2007 6:18 pm
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.
So It Goes
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by maolivie » Thu Jul 05, 2007 6:49 pm
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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