Product of slopes

maxim730 · Posted: Tue Mar 27, 2007 7:48 pm Post subject: Product of slopes

THanks in advance for your explanation.

jayhawk2001 · Posted: Tue Mar 27, 2007 10:36 pm Post subject:

Let the equation of the 2 lines l and k be

y = m1 * x + c1
y = m2 * x + c2

1 - Insufficient. Product of x intercepts = +ve.
We know that x intercept is the point at which y = 0

For line l: x = -c1 / m1
For line k: x = -c2 / m2

Product = c1c2 / m1m2 > 0

We can't infer that m1m2 > 0 since it depends on c1c2

2 - insufficient. Y intercept is -ve

For line l: y = c1
For line k: y = c2

Product = c1c2 < 0

We can't infer m1m2 from here

If we combine 1 and 2, we have c1c2 < 0 which implies that m1m2 < 0
(for c1c2/m1m2 > 0)

Hence C

I think I'm missing some datapoint here as I haven't made use of (4,3)
in the question stem...