Slope of Parallel Lines :
Theorem : If two non vertical lines are parallel, then they have the same slope.
OR
If the two lines have the same slope then the lines are parallel and non vertical.
![](figure97.gif) Figure 9.7
Slope of Perpendicular lines :
Theorem : If two non-vertical lines are perpendicular to each other then their slopes are opposite reciprocals of each other.
![](figure98.gif) Figure 9.8
If the product of slopes of two lines is -1, then the lines are non-vertical and perpendicular to each other.
Example 1
If a line l has a slope of 5/4 then (a) find the slope of a line parallel to l (b) find slope of a line perpendicular to l .
Solution :
a) The slope of line parallel to l is 5/4.
b) Let the slope of line perpendicular to be x.
![](image105.gif)
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