Slope of a Line
Here we look at graphs of straight lines in more detail and study the concept of slope
of a line.
Slope
If a highway has a 6% grade, then in 100 feet (measured horizontally) the road rises
6 feet (measured vertically). See the figure below. The ratio of 6 to 100 is 6%. If a roof rises
9 feet in a horizontal distance (or run) of 12 feet, then the roof has a 9â€“12 pitch. A
roof with a 9â€“12 pitch is steeper than a roof with a 6â€“12 pitch. The grade of a road
and the pitch of a roof are measurements of steepness. In each case the measurement
is a ratio of rise (vertical change) to run (horizontal change).
We measure the steepness of a line in the same way that we measure steepness
of a road or a roof. The slope of a line is the ratio of the change in ycoordinate, or
the rise, to the change in xcoordinate, or the run, between two points on the line.
Slope
Consider the line in the following figure. In going from (0, 1) to (1, 3),
there is a change of +1 in the xcoordinate and a change of +2 in the ycoordinate,
or a run of 1 and a rise of 2. So the slope is
or 2.
If we move from (1, 3) to (0, 1) as in the figure below, the rise is 2 and the run is
1. So the slope is
or 2.
If we start
at either point and move to the other point, we get the same slope.
Helpful Hint
Since the amount of run is arbitrary,
we can choose the run
to be 1. In this case
slope
So the slope is the amount of
change in y for a change of 1
in the xcoordinate.This is why
rates like 50 miles per hour
(mph), 8 hours per day, and
two people per car are all
slopes.
Example 1
Finding the slope from a graph
Find the slope of each line by going from point A to point B.
Solution
a) A is located at (0, 3) and B at (2, 0). In going from A to B, the change in y is
3 and the change in x is 2. So
b) In going from A(2, 1) to B(6, 3), we must rise 2 and run 4. So
c) In going from A(0, 0) to B(6, 3), we find that the rise is 3 and the run is
6. So
Note that in Example 1(c) we found the slope of the line of Example 1(b) by
using two different points. The slope is the ratio of the lengths of the two legs of a
right triangle whose hypotenuse is on the line. See the following figure.
As long as one leg is
vertical and the other leg is horizontal, all such triangles for a given line have the
same shape: They are similar triangles. Because ratios of corresponding sides in
similar triangles are equal, the slope has the same value no matter which two points
of the line are used to find it.
