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GMAT prep DS question

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Source: — Data Sufficiency |
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by atlantic » Sun Jun 29, 2008 10:45 am
Hi mlane,

The graph intersects the X axis when y=0.

So (x+a)(x+b)=0 <-> x=-a or x=-b

(1) says that a+b=-1; there are multiple choices for a em b, INSUFF

(2) the graph intersects the Y axis in (0,-6). Any graph intersects the Y axis when x=0 so a*b=-6, again there are multiple choices for a anb b.

(1+2) a+b=-1 and a*b=-6, two equations, two variables, lets see what we get...

b= 2 or b=-3
a=-3 or a=2 so the two points are (x,y)=(-3,2) and (x,y)=(2,-3)

Answer C

Hope it helps
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by Ian Stewart » Sun Jun 29, 2008 12:48 pm
atlantic wrote:(1+2) a+b=-1 and a*b=-6, two equations, two variables, lets see what we get...

b= 2 or b=-3
a=-3 or a=2 so the two points are (x,y)=(-3,2) and (x,y)=(2,-3)

Answer C
You've got the right answer to the question, but something goes a bit wrong from where I've quoted onwards. The question asks us to find the points where y = 0, so the points can't be (-3,2) and (2,-3)!

You are trying to find the values of x where (x+a)(x+b) = 0. We know a+b = -1, ab = -6, so

(x+a)(x+b) = x^2 - x - 6 = (x-3)(x+2)

And the solutions for x are x = 3, x = -2. The x-intercepts are thus (3,0) and (-2, 0).
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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