In the xy coordinate plane

In the xy-coordinate plane, line l and line k intersect at the point (4,3). Is the product of their slopes negative?
I. The product of the x-intercepts of lines l and k is positive.

II. The product of the y-intercepts of lines l and k is negative.

My Ques:  How to solve this using graphical representation?

Expert Asked on December 30, 2017 in Data Sufficiency.
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1 Answer(s)

This question becomes very easy if we use one-point form of a line.

The equation of a line is y = mx+c, where m is slope and c is y-intercept.

Now, let the equation of line l be y = mx+c and that of k be y=m1x+c1.

Therefore,

Line     x-intercept       y-intercept

l                         c/m                 c

k                        c1/m1               c1

The first statement says the product of x-intercept are +ve

i.e.      x -intercept                > 0

=>        c.c1/(m.m1) > 0 —–1

This does not give us a concrete answer as the product of slopes can be both positive or negative, depending on product of y-intercepts.

The answer will be either of B, C, E

The second statement tells us that c.c1<0.

This again is insufficient. B is eliminated.

Now, taking them together we see that if c.c1<0

then for 1 to be positive m.m1 must also be -ve.

Therefore, both together are sufficient, option C.

Expert Answered on January 19, 2018.
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