In the xy-coordinate plane, line l and line k intersect at

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In the xy-coordinate plane, line l and line k intersect at [#permalink]

18 Feb 2008, 19:01

In the xy-coordinate plane, line l and line k intersect at the point (4,3). Is the product of their slopes negative?

1) The product of the x-intercepts of lines l and k is positive.
2) The product of the y-intercepts of lines l and k is negative.

Please explain your answer

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Re: GMAT Prep: XY-Coordinate Plane Ques [#permalink]

19 Feb 2008, 23:15

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C

To make using this editor easy, I am naming lines as 1 and 2
and thier x and y intercepts as x1, x2, y1 and y2

slope of line 1 = -y1/x1
slope of line 2 = -y2/x2

product of slopes = (y1*y2)/(x1*x2)
since numerator is -ve and denominator is +ve, product of slopes is -ve

What is OA?

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Re: GMAT Prep: XY-Coordinate Plane Ques [#permalink]

20 Feb 2008, 11:53

Ohhhhhhhhhhhhhhh!!!!!!!!!! Thanks sreehari! I remember I got raped by this question on GMATPrep, I was trying to draw lines out over and over, lol! :-)

That's so simple.

So line 1's slope is -y1/x1 & line 2's slope is -y2/x1

So the product of their slopes is y1*y2 / (x1*x2)

And if you take the stuff together, ah!!!!!!! Nice :-)

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Re: GMAT Prep: XY-Coordinate Plane Ques [#permalink]

05 Jun 2011, 11:02

I still dont get it. Can someone please further explain in a more simple way?
Many thanks.

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Re: GMAT Prep: XY-Coordinate Plane Ques [#permalink]

06 Jun 2011, 00:06

its ok now, i just got confused by the -y1 and -y2. thought its a negative sign.

Thanks

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Re: GMAT Prep: XY-Coordinate Plane Ques [#permalink]

07 Jun 2011, 11:14

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bsjames2 wrote:

This question has a great takeaway - something I am sure you know intuitively but you may not think of it while doing this question because it is seldom written out:
The slope of a line is -(y intercept)/(x intercept)

Above, sreehari uses this concept to solve the question very efficiently.

If you are wondering why it is so, think what 'intercept' represents...
The point of x intercept is (x, 0) (where y co-ordinate is 0)
The point of y intercept is (0, y) (where x co-ordinate is 0)

So slope = (y - 0)/(0 - x) = -y/x
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Re: GMAT Prep: XY-Coordinate Plane Ques [#permalink]

10 Jun 2011, 19:35

I answered correctly but after drawing lines all over....thanks Karishma for the explanation
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Re: GMAT Prep: XY-Coordinate Plane Ques [#permalink] 10 Jun 2011, 19:35

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