Horizontal and Vertical Lines
The slope of a horizontal line is neither positive nor negative.
The slope of a vertical line is undefined. That is, a vertical line does not
have a slope.
Example 1
Find the slope of the horizontal line through points (-4, 2) and (3, 2).
Solution
| Let (x1, y1) = (-4, 2) and (x2,
y2) = (3, 2). |
m |
 |
| Substitute the values in the slope formula. |
|
 |
| Simplify. |
|
= 0 |
The slope of the line is 0.
To move from one point to another on a horizontal line, the rise is 0.
So the slope of a horizontal line is 0.
Example 2
Find the slope of the vertical line that passes through (3, -4) and (3, 2).
Solution
| Let (x1, y1) = (3, -4) and (x2,
y2) = (3, 2). |
m |
|
| Substitute the values in the slope formula.
Simplify. |
|
 |
Division by 0 is undefined, so the slope of the line is undefined.
To move from one point to another on a vertical line, the run is 0. The
slope of a vertical line is undefined. That is, a vertical line does not have
a slope.
Note - Slope of a Line
| • Positive slope: Line slants
upward as we move from left to
right.
• Negative slope: Line slants
downward as we move from left
to right.
• Zero slope: A horizontal line
has slope 0.
• Undefined slope: The slope of
a vertical line is undefined. |
 |
|
