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

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Director
Joined: 11 Jun 2007
Posts: 637
In the xy-coordinate plane, line l and line k intersect at [#permalink]

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11 Nov 2007, 10:57
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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

Explanations required .
Senior Manager
Joined: 19 Feb 2007
Posts: 325

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11 Nov 2007, 11:07
Let the 2 eqations be

y1 = (m1 * x1) + c1
y2 = (m2 * x2) + c2

From 1

X intercept is x at y=0

so

x1 = -c1/x1
x2 = -c2/x2

x1 * x2 = (c1 * c2)/(m1*m2) is +ve

Insuff.

Doesn't tell us whether m1*m2 is +ve or -ve

From 2

c1 * c2 is -ve.

Insuff.

Doesn't tell us whether m1*m2 is +ve or -ve

Both:
Suff.

m1*m2 is -ve

So the ans is (C)
11 Nov 2007, 11:07
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