1. Parallel & Perpendicular Lines
Writing Equations of Parallel and Perpendicular Lines

2. Objectives Vocabulary parallel lines perpendicular lines
Identify and graph parallel and perpendicular lines.
Write equations to describe lines parallel or perpendicular to a given line.
Vocabulary
parallel lines
perpendicular lines

3. Parallel and Perpendicular
Remember parallel lines have the same slopes so if you need the slope of a line parallel to a given line, simply find the slope of the given line and the slope you want for a parallel line will be the same.
Perpendicular lines have negative reciprocal slopes so if you need the slope of a line perpendicular to a given line, simply find the slope of the given line, take its reciprocal (flip it over) and make it negative.

5. Helpful Hint
If you know the slope of a line, the slope of a perpendicular line will be the "opposite reciprocal.”

6. Writing Equations of Parallel and Perpendicular Lines
Write an equation in slope-intercept form for the line that passes through (4, 10) and is parallel to the line described by y = 3x + 8.
Step 1 Find the slope of the line.
y = 3x + 8
The slope is 3.
The parallel line also has a slope of 3.
Step 2 Write the equation in point-slope form.
y – y1 = m(x – x1)
Use the point-slope form.
Substitute 3 for m, 4 for x1, and 10 for y1.
y – 10 = 3(x – 4)

7. Continued
Write an equation in slope-intercept form for the line that passes through (4, 10) and is parallel to the line described by y = 3x + 8.
Step 3 Write the equation in slope-intercept form.
y – 10 = 3(x – 4)
y – 10 = 3x – 12)
Distribute 3 on the right side.
y = 3x – 2
Add 10 to both sides.

8. Writing Equations of Parallel and Perpendicular Lines
Write an equation in slope-intercept form for the line that passes through (2, –1) and is perpendicular to the line described by y = 2x – 5.
Step 1 Find the slope of the line.
y = 2x – 5
The slope is 2.
The perpendicular line has a slope of because
Step 2 Write the equation in point-slope form.
y – y1 = m(x – x1)
Use the point-slope form.
Substitute for m, –1 for y1, and 2 for x1.

9. Step 3 Write the equation in slope-intercept form.
Continued
Write an equation in slope-intercept form for the line that passes through (2, –1) and is perpendicular to the line described by y = 2x – 5.
Step 3 Write the equation in slope-intercept form.
Distribute on the right side.
Subtract 1 from both sides.

10. Try This!
Write an equation in slope-intercept form for the line that passes through (5, 7) and is parallel to the line described by y = x – 6.
Step 1 Find the slope of the line.
The slope is .
y = x –6
The parallel line also has a slope of
Step 2 Write the equation in point-slope form.
y – y1 = m(x – x1)
Use the point-slope form.

11. Try This! Continued
Write an equation in slope-intercept form for the line that passes through (5, 7) and is parallel to the line described by y = x – 6.
Step 3 Write the equation in slope-intercept form.
Distribute on the right side.
Add 7 to both sides.

12. Try This!
Write an equation in slope-intercept form for the line that passes through (–5, 3) and is perpendicular to the line described by y = 5x.
Step 1 Find the slope of the line.
y = 5x
The slope is 5.
The perpendicular line has a slope of because .
Step 2 Write the equation in point-slope form.
y – y1 = m(x – x1)
Use the point-slope form.

13. Try This! Continued
Write an equation in slope-intercept form for the line that passes through (–5, 3) and is perpendicular to the line described by y = 5x.
Step 3 Write in slope-intercept form.
Distribute on the right side.
Add 3 to both sides.

14. Write an equation is slope-intercept form for the line described.
Try This!
Write an equation is slope-intercept form for the line described.
1. contains the point (8, –12) and is parallel to
2. contains the point (4, –3) and is perpendicular to y = 4x + 5