4.. Please help. Is DE←→ perpendicular to FG←→? Why or why not?

Given:

DE contains the points D(1, -2) and E(3, 4).

FG contains the points F(-1, 2) and G(4, 0).

To find:

Is DE perpendicular to FG.

Solution:

Slope of DE:

Here

m = 3

Slope of FG:

Here

Two lines are perpendicular if product their slopes are -1.

Slope of DE × Slope of FG

         ≠ -1

The solution is no, because the product of the slopes is not -1.