1. Lesson 5.5 Write Equations of Parallel and Perpendicular Lines
Essential Question: How do you write equations of parallel and perpendicular lines?

2. Before we start…
The public library is located on Adams Street at the point (3, 2). The post office is also located on Adams Street at the point (5, 3). Write and graph an equation of the line that represents Adams Street.
Walnut Street is parallel to Adams’ Street. The high school is located on Walnut Street at the point (2, 5). Do you have enough information to write an equation of the line that represents Walnut Street?

3. What is special about parallel lines?
Recall that if two non vertical lines have the same slope, then they are parallel.
So parallel lines never intersect when graphed on a coordinate plane.

4. Two Conditional Statements about Parallel Lines
If two non vertical lines in the same plane have the same slope, then they are parallel.
If two non vertical lines in the same plane are parallel, then they have the same slope.

5. How do you write the equation of a parallel line?
First identify the slope.
Use the slope and the given point in point slope form.
Convert to slope-intercept form.

10. What about perpendicular lines?
Two lines in the same plane are perpendicular if they intersect to form a right angle.

11. Two Conditional Statements about Perpendicular Lines
If two non vertical lines in the same plane have slopes that are negative reciprocals, then the lines are perpendicular.
If two non vertical lines in the same plane are perpendicular, then their slopes are negative reciprocals.

12. Determine which lines, if any, are parallel or perpendicular.

13. Determine which lines, if any, are parallel or perpendicular.

14. How do you write the equation of a perpendicular line?
Identify the slope.
Take the negative reciprocal of the slope of the given line.
Use the slope and the given point in point slope form.
Convert to slope-intercept form.

19. Parallel and Perpendicular Lines
Parallel Lines
Have the SAME slope
Perpendicular Lines
Have the NEGATIVE RECIPROCAL slope

20. How do you write equations of parallel and perpendicular lines?

21. Ticket Out the Door