Is the answer right? : GMAT Quantitative Section
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31 May 2013, 20:03
In the XY Plane, what two points does the Y=(x+a)(x+b) intersect the X axis?

1. A+B = -1
2. the graph intersects y axis at (0,-6)

Answer is Both Statements together are sufficient, but neither statements alone are.

Shouldn't the answer be 1&2 together arent sufficient?
It is not possible to decide conclusively which of A & B are -3,2
thus leaving 2 answers for each variables.
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31 May 2013, 20:52
y = (x+a)(x+b) = x^2 + x (a+b) +ab. -a, -b are the two points where the curve intersects x-axis

From Stmt 1, a + b = -1

a = -7, b = 6 or a = 6, b = -7 Not Sufficient

From Stmt 2, substituting point (0, -6) in y = x^2 + x(a+b) + ab => -6 = ab

a = -6, b = 1 or a = 1, b = -6 Not Sufficient

Taking Stmt 1 and Stmt 2

ab = -6 and a+b = -1
-b^2 -b + 6 = 0 => (b + 3)(b - 2) = 0 => b = -3 or 2 => a = 2, -3. Both these values lead to the same two points - 3 and -2 Therefore answer is C.

Hope this helps
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01 Jun 2013, 02:40
harryyou wrote:
In the XY Plane, what two points does the Y=(x+a)(x+b) intersect the X axis?

1. A+B = -1
2. the graph intersects y axis at (0,-6)

Answer is Both Statements together are sufficient, but neither statements alone are.

Shouldn't the answer be 1&2 together arent sufficient?
It is not possible to decide conclusively which of A & B are -3,2
thus leaving 2 answers for each variables.

This question is discussed here: in-the-xy-plane-at-what-points-does-the-graph-of-y-88398.html OA is C.

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