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 (x_{1}, y_{1}) = (4, 2) and (x_{2},
y_{2}) = (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 (x_{1}, y_{1}) = (3, 4) and (x_{2},
y_{2}) = (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
â€¢ Positive slope: Line slants
upward as we move from left to
right.
â€¢ Negative slope: Line slants
downward as we move from left
to right.
â€¢ Zero slope: A horizontal line
has slope 0.
â€¢ Undefined slope: The slope of
a vertical line is undefined. 

