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?
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.