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In the xy-coordinate plane, , line l and line k intersect at [#permalink]
 22 Apr 2006, 09:27
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In the xy-coordinate plane, , line l and line k intersect at point (4,3). Is the product of their slopes negative?
1) The product of the x-intercepts of the lines l & K is positive.
2) The product of the y-intercepts of the lines l & K is negative.
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Equation of line in Intercept form x/a + y/b = 1 where x intercept is a and y intercept is b. Slope of the line is -b/a.
Let the two lines in this form be x/a1 + y/b1 = 1 and x/a2 + y/b2 = 1.
Then the product of the slopes is -(b1/a1) * -(b2/a2) = b1*b2/a1*a2
First condition gives a1*a2>0
2nd one gives b1*b2 <0
Hence together are necessary to conclude b1*b2/a1*a2 <0
The answer is C
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OA is C. Thanks.
The information about Point (4,3) seems redundant.
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