5.5: Families of Lines
What if you were given the equation of a line like and you wanted to find the equation of a line that is parallel or perpendicular to it that passes through the point (2, 1). How could you find the equation of this line? After completing this Concept, you'll be able to write equations of perpendicular and parallel lines.
Watch This
CK12 Foundation: 0505S Equations of Parallel and Perpendicular Lines (H264)
Guidance
We can use the properties of parallel and perpendicular lines to write an equation of a line parallel or perpendicular to a given line. You might be given a line and a point, and asked to find the line that goes through the given point and is parallel or perpendicular to the given line. Here’s how to do this:
 Find the slope of the given line from its equation. (You might need to rewrite the equation in a form such as the slopeintercept form.)
 Find the slope of the parallel or perpendicular line—which is either the same as the slope you found in step 1 (if it’s parallel), or the negative reciprocal of the slope you found in step 1 (if it’s perpendicular).
 Use the slope you found in step 2, along with the point you were given, to write an equation of the new line in slopeintercept form or pointslope form.
Example A
Find an equation of the line perpendicular to the line that passes through the point (2, 6).
Solution
The slope of the given line is 3, so the perpendicular line will have a slope of .
Now to find the equation of a line with slope that passes through (2, 6):
Start with the slopeintercept form: .
Plug in the slope: .
Plug in the point (2, 6) to find : .
The equation of the line is .
Example B
Find the equation of the line parallel to that passes through the point (5, 3).
Solution
Rewrite the equation in slopeintercept form: .
The slope of the given line is , so we are looking for a line with slope that passes through the point (5, 3).
Start with the slopeintercept form: .
Plug in the slope: .
Plug in the point (5, 3):
The equation of the line is .
Investigate Families of Lines
A family of lines is a set of lines that have something in common with each other. Straight lines can belong to two types of families: one where the slope is the same and one where the intercept is the same.
Family 1: Keep the slope unchanged and vary the intercept.
The figure below shows the family of lines with equations of the form :
All the lines have a slope of –2, but the value of is different for each line.
Notice that in such a family all the lines are parallel. All the lines look the same, except that they are shifted up and down the axis. As gets larger the line rises on the axis, and as gets smaller the line goes lower on the axis. This behavior is often called a vertical shift.
Family 2: Keep the intercept unchanged and vary the slope.
The figure below shows the family of lines with equations of the form :
All the lines have a intercept of two, but the slope is different for each line. The steeper lines have higher values of .
Example C
Write the equation of the family of lines satisfying the given condition.
a) parallel to the axis
b) through the point (0, 1)
c) perpendicular to
d) parallel to
Solution
a) All lines parallel to the axis have a slope of zero; the intercept can be anything. So the family of lines is or just .
b) All lines passing through the point (0, 1) have the same intercept, . The family of lines is: .
c) First we need to find the slope of the given line. Rewriting in slopeintercept form, we get . The slope of the line is , so we’re looking for the family of lines with slope .
The family of lines is .
d) Rewrite in slopeintercept form: . The slope is , so that’s also the slope of the family of lines we are looking for.
The family of lines is .
Watch this video for help with the Examples above.
CK12 Foundation: Equations of Parallel and Perpendicular Lines
Vocabulary
 A family of lines is a set of lines that have something in common with each other. Straight lines can belong to two types of families: one where the slope is the same and one where the intercept is the same.
 Notice that in such a family all the lines are parallel. All the lines look the same, except that they are shifted up and down the axis. As gets larger the line rises on the axis, and as gets smaller the line goes lower on the axis. This behavior is often called a vertical shift.
Guided Practice
Find the equation of the line perpendicular to that passes through the point (2, 5).
Solution
Rewrite the equation in slopeintercept form: .
The slope of the given line is , so we’re looking for a line with slope 5.
Start with the slopeintercept form: .
Plug in the slope: .
Plug in the point (2, 5):
The equation of the line is .
Explore More
 Find the equation of the line parallel to that passes through point (3, 2).
 Find the equation of the line perpendicular to that passes through point (2, 8).
 Find the equation of the line parallel to that passes through the point (2, 2).
 Find the equation of the line perpendicular to that passes through the point (6, 2).
 Line passes through the points (2, 3) and (4, 7). Line passes through the point (2, 5). If Lines and are parallel, name one more point on line . ( Hint: you don’t need to find the slope of either line.)
 Lines and both pass through (1, 5). Line also passes through (3, 1). If and are perpendicular, name one more point on line . (This time you will have to find the slopes of both lines.)

Write the equation of the family of lines satisfying the given condition.
 All lines that pass through point (0, 4).
 All lines that are perpendicular to .
 All lines that are parallel to .
 All lines that pass through the point (0, 1).
 Name two lines that pass through the point (3, 1) and are perpendicular to each other.
 Name two lines that are each perpendicular to . What is the relationship of those two lines to each other?
 Name two perpendicular lines that both pass through the point (3, 2). Then name a line parallel to one of them that passes through the point (2, 5).
Image Attributions
Description
Learning Objectives
Here you'll learn how to write the equation of a line that is parallel or perpendicular to a second line given that second line's equation and one of the points it passes through. You'll also investigate families of lines.
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Date Created:
Aug 13, 2012Last Modified:
Oct 28, 2014Vocabulary
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