In addition to its familiar meaning, the word "slope" has precise mathematical meaning. The slope of a line is the rise over the run, or the change in
*y*
divided by the change in
*x*
. To find the slope of a line, pick *any* two points on the line. Then subtract their x-coordinates and subtract their y-coordinates **in the same order**. Divide the difference of the
*y*
-coordinates by the difference of the
*x*
- coordinates:

Given two points (x_{1},y_{1}) and (x_{2},y_{2}) on a line, the slope of the line is equal to:

m= =

*Example 1.* Find the slope of the line which passes through the points
(2, 5)
and
(0, 1)
:

Slope

If a line has a positive slope (i.e.
*m* > 0
), then
*y*
always increases when
*x*
increases and
*y*
always decreases when
*x*
decreases. Thus, the graph of the line starts at the bottom left and goes towards the top right.

Often, however, the slope of a line is negative. A negative slope implies that
*y*
always decreases when
*x*
increases and
*y*
always increases when
*x*
decreases. Here is an example of a graph with negative slope:

Negative Slope

Thus, as

Sometimes, we will see equations whose graphs are horizontal lines. These are graphs in which
*y*
remains constant -- that is, in which
*y*
_{1} - *y*
_{2} = 0
for any two points on the line:

Graph of a Horizontal Line

We will also see equations whose graphs are vertical lines. These are graphs in which
*x*
remains constant -- that is, in which
*x*
_{1} - *x*
_{2} = 0
for any two points on the line:

Graph of a Vertical Line

Intuitively, it makes sense that one cannot assign a slope to a vertical line; a line that is "almost" vertical (i.e. is very steeply inclined) could have a very large positive *or* negative slope. So there is no way to decide even whether or not a vertical line should have positive or negative slope (and it clearly cannot have zero slope).

Two lines are parallel if they have the same slope. Parallel lines, when extended, do not intersect at any point.

Graph of Parallel Lines

Two lines are perpendicular if their slopes are opposite reciprocals of each other. For example, if a line has a slope of , a perpendicular line has a slope of - . Perpendicular lines intersect each other at right angles.

Graph of Perpendicular Lines