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In the xy-plane, at what two points does the graph of y =

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In the xy-plane, at what two points does the graph of y = [#permalink]

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In the xy-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 intersect the y-axis at (0,-6)

OPEN DISCUSSION OF THIS QUESTION IS HERE: http://gmatclub.com/forum/in-the-xy-pla ... 88398.html
[Reveal] Spoiler: OA

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Re: In the xy-plane, at what two points does the graph of y = [#permalink]

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New post 23 Apr 2011, 16:11
agdimple333 wrote:
In the xy-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 intersect the y-axis at (0,-6)


Clearly the graph intersects at (-a,0) and (-b,0), so we need to find value of a and b.

Statement 1 gives a+b = -1, so not sufficient

Statement 2 says that y = -6 when x is 0 or ab=-6, so not sufficient.

Combining 1 and 2, we can get that a and b are -3 and 2 respectively, so sufficient.

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Re: In the xy-plane, at what two points does the graph of y = [#permalink]

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New post 24 Apr 2011, 22:09
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y = (x+a)(x+b) intersect the x-axis when y = 0, which happens when x = -a or x = -b

=> 0 = (x+a)(x+b)

x^2 + x(a+b) + ab = 0

(1) gives us value of (a+b), but not that of ab

So insufficient.

(2) says

-6 = (0+a)(0+b) => ab = -6


Does not give us value of (a+b), so insufficient.

(1) and (2) give x^2 -x - 6 = 0

(x-3)(x+2) = 0

=> x = 3, -2

Sufficient

Answer - C.
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Re: In the xy-plane, at what two points does the graph of y = [#permalink]

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Re: In the xy-plane, at what two points does the graph of y = [#permalink]

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New post 15 Aug 2017, 10:24
st 2 tells that the peak of the parabol is fixed, and st 1 indicates that the intersected points are fixed => C is the answer. There is no need to check whether the values of the equatation.

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Re: In the xy-plane, at what two points does the graph of y = [#permalink]

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New post 03 Oct 2017, 09:19
agdimple333 wrote:
In the xy-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 intersect the y-axis at (0,-6)

OPEN DISCUSSION OF THIS QUESTION IS HERE: http://gmatclub.com/forum/in-the-xy-pla ... 88398.html


OPEN DISCUSSION OF THIS QUESTION IS HERE: http://gmatclub.com/forum/in-the-xy-pla ... 88398.html
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Re: In the xy-plane, at what two points does the graph of y =   [#permalink] 03 Oct 2017, 09:19
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