In the xy-plane, at what two points does the graph of : DS Archive
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# In the xy-plane, at what two points does the graph of

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

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07 Jun 2008, 17:13
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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 intersects the y-axis at (0,-6)

peer
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Re: DS 2 - Line interesection [#permalink]

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07 Jun 2008, 19:26
E

The equation y = (x+a)(x+b) expands into a quadratic with the roots -a and -b. So the question is asking what are the values of a and b and hence the points (a, 0) and (b, 0) are the 2 points of intersection with the x axis.

1. a+b=-1, insuff to find a and b
2. this leads to ab = -6, insuff

1+2
solving for a and b we get -2 and 3 for b and 3 and -2 for a. So, there are multiple possibilities for a and b. Hence E
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Re: DS 2 - Line interesection [#permalink]

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07 Jun 2008, 22:53
peergmatclub 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 intersects the y-axis at (0,-6)

peer

C

y=x^2+x(a+b)+ab----------(1)

1)a+b=-1
2)put x=0 in (1) ab=-6

combine

y=x^2+x(-1)-6
for a lne to touch x xis y=0

0=x^2-x-6

solving x=-2,3

points=(-2,0),(3,0)
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Re: DS 2 - Line interesection [#permalink]

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08 Jun 2008, 15:06
Found my mistake....thanks Rohit...
Re: DS 2 - Line interesection   [#permalink] 08 Jun 2008, 15:06
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