Which of the following best describes perpendicular lines?

a. lines that are not in the same plane

b. lines that meet at a 90 degree angle

c. lines that meet at a 45 degree angle

d. lines that are coplanar and do not intersect

Lines that meet as a 90° angle.

...

For lines to be perpendicular, the slopes of the lines must be negative reciprocals of one another, mathematically:

m1*m2=-1

Note that "slope" is velocity or change in y divided by change in x.  Using any value for m1, say 3 as an example.

m1=3, then m2=-1/3

tanα=dy/dx=m so

Then if we use the arctan function we can see the angles of the lines, ie

α=arctan3, ß=arctan(-1/3)

α≈71.56°, ß≈-18.44

α-ß=90°