What is the definition of a slope of parallel lines?

Answer 1

Horizontal lines have a slope of zero, and the slope of vertical lines is undefined.

Parallel lines have equal slopes, and perpendicular lines have slopes that are negative reciprocals of each other.

So we can say that:

Two nonvertical lines are parallel if and only if they have the same slope.

Two lines are perpendicular if and only if their slopes are negative reciprocals of each other. That is, if the slopes are m1 and m2, then:

m1 = - 1/m2 or (m1)(m2) = -1