ORCCA Geometry of Lines

Section 3.8 Geometry of Lines

Objectives: PCC Course Content and Outcome Guide

Figure 3.8.1. Alternative Video Lessons

The equations of horizontal and vertical lines are special and distinguished from the equations other lines. Also, pairs of lines that are parallel or perpendicular to each other have interesting features and properties. This section examines all these geometric features of lines.

the graph of a horizontal line passing through the point (0,3) — Figure 3.8.2. Horizontal Line

the graph of a vertical line passing through the point (3,0) — Figure 3.8.3. Vertical Line

the graph of two parallel slanted lines — Figure 3.8.4. Two Parallel Lines

the graph of a slanted line and the line that crosses it at a 90 degree angle — Figure 3.8.5. Two Perpendicular Lines

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Subsection 3.8.1 Horizontal Lines and Vertical Lines

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We learned in Section 7 that all lines can be written in standard form. When either

A

B

equal

0,

we end up with a horizontal or vertical line. Let’s take the standard form line equation,

A x + B y = C,

and one at a time let

A = 0

and

B = 0

and simplify each equation.

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\begin{aligned} A x + B y & = C & A x + B y & = C \\ 0 x + B y & = C & A x + 0 y & = C \\ B y & = C & A x & = C \\ y & = \frac{C}{B} & x & = \frac{C}{A} \\ y & = k & x & = h \end{aligned}

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At the end we just renamed the constant numbers

\frac{C}{B}

and

\frac{C}{A}

k

and

h

because of tradition. What is important, is that you view

h

and

k

(as well as

A,

B,

and

C

) as constants: numbers that have some specific value and don’t change.

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Think about one of these equations:

y = k .

It says that the

y

-value is the same no matter where you are on the line. If you wanted to plot points on this line, you are free to move to the left or to the right on the

x

-axis, but then you always move up (or down) by the same amount to make the

y

-value reach

k .

What does such a line look like?

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Example 3.8.6.

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Let’s plot the line with equation

y = 3 .

To plot some points, it doesn’t matter what

x

-values we use. All that matters is that

y

is always

3 .

a horizontal line at y=3 — Figure 3.8.7. $y = 3$

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A line like the one in Figure 7 is horizontal. It is parallel to the horizontal axis. All the

y

-values of points on a horizontal line are the same. All lines with an equation in the form

y = k

are horizontal lines.

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Example 3.8.8.

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Let’s plot the line with equation

x = 5 .

Points on the line always have

x = 5,

so to make a graph, we must move right to

5

on the

x

-axis. From there, it does not matter if we move up or down, we would still be at a place where

x = 5 .

a vertical line at x=5 — Figure 3.8.9. $x = 5$

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A line like this is vertical, parallel to the vertical axis. All lines with an equation in the form

x = h

(or, in standard form,

A x + 0 y = C

) are vertical.

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A line like the one in Figure 9 is vertical. It is parallel to the vertical axis. All the

y

-values of points on a vertical line are the same. All lines with an equation in the form

x = h

are vertical lines.

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Example 3.8.10. Zero Slope.

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In Checkpoint 3.4.18, we learned that a horizontal line’s slope is

0,

because the

y

-values don’t change as time moves on. So the numerator in the slope formula is

0 .

Now, if we know a line’s slope and its

y

-intercept, we can use slope-intercept form to write its equation:

\begin{aligned} y & = m x + b \\ y & = 0 x + b \\ y & = b \end{aligned}

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This gives us an alternative way to think about equations of horizontal lines. They have a certain

y

-intercept

b,

and they have slope

0 .

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We use horizontal lines to model scenarios where there is no change in

y

-values, like when Kato stopped for

12

hours (he deserved a rest)!

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Checkpoint 3.8.11. Plotting Points.

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Suppose you need to plot the equation

y = - 4.25 .

Since the equation is in “

y =

” form, you decide to make a table of points. Fill out some points for this table.

$x$	$y$

Explanation.

We can use whatever values for

x

that we like, as long as they are all different. The equation tells us the

y

-value has to be

- 4.25

each time.

$x$	$y$
$- 2$	$- 4.25$
$- 1$	$- 4.25$
$0$	$- 4.25$
$1$	$- 4.25$
$2$	$- 4.25$

Now that we have a table, we could use its values to assist with plotting the line.

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Example 3.8.12. Slope of a Vertical Line.

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What is the slope of a vertical line? Figure 13 shows three lines passing through the origin, each steeper than the last. In each graph, you can see a slope triangle that uses a “run” of

1

unit.

a line with a slope of 1/1=1 — Figure 3.8.13.

a line with a slope of 2/1=2, which is steeper than the first line — Figure 3.8.13.

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If we continued making the line steeper and steeper until it was vertical, the slope triangle would still have a “run” of

1,

but the “rise” would become larger and larger with no upper limit. The slope would be

m = \frac{very large}{1} .

Actually if the line is vertical, the “rise” segment we’ve drawn will never make contact with the line. So there won’t be any “rise” to correspond with that “run”. We usually say that the slope of a vertical line is undefined. You can also say that a vertical line “has no slope”.

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Fact 3.8.14.

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The slope of a vertical line is undefined.

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Remark 3.8.15.

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Be careful not to mix up “no slope” (which means “its slope is undefined”) with “has slope

0

”. If a line has slope

0,

it does have a slope.

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In sports, some players wear number

0 .

That’s not the same thing as not having a number. This is similar to the situation where having slope

0

means you do have a slope, and is different from not having a slope.

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Checkpoint 3.8.16. Plotting Points.

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Suppose you need to plot the equation

x = 3.14 .

You decide to try making a table of points. Fill out some points for this table.

$x$	$y$

Explanation.

Since the equation says

x

is always the number

3.14,

we have to use this for the

x

value in all the points. This is different from how we would plot a “

y =

” equation, where we would use several different

x

-values. We can use whatever values for

y

that we like, as long as they are all different.

$x$	$y$
$3.14$	$- 2$
$3.14$	$- 1$
$3.14$	$0$
$3.14$	$1$
$3.14$	$2$

The reason we made a table was to help with plotting the line.

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Example 3.8.17.

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Let

x

represent the price of a new

60

-inch television at Target on Black Friday (which was

$ 650

), and let

y

be the number of hours you will watch something on this TV over its lifetime. What is the relationship between

x

and

y ?

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Well, there is no getting around the fact that

x = 650 .

As for

y,

without any extra information about your viewing habits, it could theoretically be as low as

0

or it could be anything larger than that. If we graph this scenario, we have to graph the equation

x = 650

which we now know to give a vertical line, and we get Figure 18.

$the vertical line x=650, starting at the point (650,0) with an arrow pointing upward$

Figure 3.8.18. New TV: hours watched versus purchase price; negative

y

-values omitted since they make no sense in context

Horizontal Lines	Vertical Lines
🔗 🔗 A line is horizontal if and only if its equation can be written $y = k$ 🔗 for some constant $k .$	🔗 🔗 A line is vertical if and only if its equation can be written $x = h$ 🔗 for some constant $h .$
🔗 🔗 In standard form, any line with equation $0 x + B y = C$ 🔗 is horizontal.	🔗 🔗 In standard form, any line with equation $A x + 0 y = C$ 🔗 is vertical.
🔗 If the line with equation $y = k$ is horizontal, it has a $y$ -intercept at $(0, k)$ and has slope $0 .$	🔗 If the line with equation $x = h$ is vertical, it has an $x$ -intercept at $(h, 0)$ and its slope is undefined. Some say it has no slope, and some say the slope is infinitely large.
🔗 🔗 In the slope-intercept form, any line with equation $y = 0 x + b$ 🔗 is horizontal.	🔗 It’s impossible to write the equation of a vertical line in slope-intercept form, because vertical lines do not have a defined slope.

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Figure 3.8.19. Summary of Horizontal and Vertical Line Equations

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Subsection 3.8.2 Parallel Lines

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What makes two lines parallel?

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Example 3.8.20.

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Two trees were planted in the same year, and their growth over time is modeled by the two lines in Figure 21. Use linear equations to model each tree’s growth, and interpret their meanings in this context.

This is a Cartesian grid with two parallel lines. Tree 1 has a slope triangle from (0,2) to (3,4) for a slope of 2/3. Tree 2 has a slope triangle from (0,5) to (3.7) for a slope of 2/3. — Figure 3.8.21. Two Trees’ Growth Chart

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We can see Tree 1’s equation is

y = \frac{2}{3} x + 2,

and Tree 2’s equation is

y = \frac{2}{3} x + 5 .

Both trees have been growing at the same rate,

\frac{2}{3}

feet per year, or

2

feet every

3

years. The two lines have the same slope

\frac{2}{3} .

No matter which line we look at, moving rightward

3

units causes us to move upward

2

units, and so the two lines will never meet. They are parallel.

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Fact 3.8.22.

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For any two non-vertical lines, they are parallel if and only if they have the same slope. (And any two vertical lines are parallel to each other.)

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Checkpoint 3.8.23.

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A line

ℓ

is parallel to the line with equation

y = 17.2 x - 340.9,

but

ℓ

has

y

-intercept at

(0, 128.2) .

What is an equation for

ℓ ?

Explanation.

Parallel lines have the same slope, and the slope of

y = 17.2 x - 340.9

17.2 .

ℓ

has slope

17.2 .

And we have been given that

ℓ

’s

y

-intercept is at

(0, 128.2) .

So we can use slope-intercept form to write its equation as

y = 17.2 x + 128.2 .

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Checkpoint 3.8.24.

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A line

κ

is parallel to the line with equation

y = - 3.5 x + 17,

but

κ

passes through the point

(- 12, 23) .

What is an equation for

κ ?

Explanation.

Parallel lines have the same slope, and the slope of

y = - 3.5 x + 17

- 3.5 .

κ

has slope

- 3.5 .

And we know a point that

κ

passes through, so we can use point-slope form to write its equation as

y = - 3.5 (x + 12) + 23 .

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Subsection 3.8.3 Perpendicular Lines

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The slopes of two perpendicular lines have a special relationship too. Figure 25 walks you through an explanation of this relationship.

the graph of two perpendicular lines, where one has a slope of m — (a) Two generic perpendicular lines, where one has slope $m .$

the previous graph with a slope triangle added; the rise is m and the run is 1 — (a) Two generic perpendicular lines, where one has slope $m .$

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When the first line in Figure 25 has slope

m,

the second has slope

\frac{Δ y}{Δ x} = \frac{- 1}{m} = - \frac{1}{m} .

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Fact 3.8.26.

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For two lines that are neither vertical nor horizontal, they are perpendicular to each other if and only if the slope of one is the negative reciprocal of the slope of the other. That is, if one has slope

m,

the other has slope

- \frac{1}{m} .

(And a vertical line and a horizontal line are always perpendicular to each other.)

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Another way to say this is that the product of the slopes of two perpendicular lines is

- 1

(assuming both of the lines have a slope in the first place). That is, if there are two perpendicular lines and we let

m_{1}

and

m_{2}

represent their slopes, then

m_{1} \cdot m_{2} = - 1 .

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Here are three pairs of perpendicular lines where we can see if the pattern holds.

the graph of lines y=2x-2 and y=-1/2x+2 showing that they are perpendicular lines. — Figure 3.8.27. $y = 2 x - 2$ and $y = - \frac{1}{2} x + 2 .$ Note the relationship between their slopes: $2 \cdot - \frac{1}{2} = - 1$

the graph of lines y=-3x+4 and y=1/3x-3 showing that they are perpendicular — Figure 3.8.28. $y = - 3 x + 4$ and $y = \frac{1}{3} x - 3 .$ Note the relationship between their slopes: $- 3 \cdot \frac{1}{3} = - 1$

the graph of lines y=x and y=-x showing that they are perpendicular — Figure 3.8.29. $y = x$ and $y = - x .$ Note the relationship between their slopes: $1 \cdot - 1 = - 1$

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Example 3.8.30.

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Line

A

passes through

(- 2, 10)

and

(3, - 10) .

Line

B

passes through

(- 4, - 4)

and

(8, - 1) .

Determine whether these two lines are parallel, perpendicular or neither.

Explanation.

We will use the slope formula to find both lines’ slopes:

\begin{aligned} Line A's slope & = \frac{y_{2} - y_{1}}{x_{2} - x_{1}} & Line B's slope & = \frac{y_{2} - y_{1}}{x_{2} - x_{1}} \\ = \frac{- 10 - 10}{3 - (- 2)} & = \frac{- 1 - (- 4)}{8 - (- 4)} \\ = \frac{- 20}{5} & = \frac{3}{12} \\ = - 4 & = \frac{1}{4} \end{aligned}

Their slopes are not the same, so those two lines are not parallel.

The product of their slopes is

(- 4) \cdot \frac{1}{4} = - 1,

which means the two lines are perpendicular.

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Checkpoint 3.8.31.

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Line

A

and Line

B

are perpendicular. Line

A

’s equation is

2 x + 3 y = 12 .

Line

B

passes through the point

(4, - 3) .

Find an equation for Line

B .

Explanation.

First, we will find Line

A

’s slope by rewriting its equation from standard form to slope-intercept form:

\begin{aligned} 2 x + 3 y & = 12 \\ 3 y & = 12 - 2 x \\ 3 y & = - 2 x + 12 \\ y & = \frac{- 2 x + 12}{3} \\ y & = - \frac{2}{3} x + 4 \end{aligned}

So Line

A

’s slope is

- \frac{2}{3} .

Since Line

B

is perpendicular to Line

A,

its slope is the negative reciprocal,

\frac{3}{2} .

It’s also given that Line

B

passes through

(4, - 3),

so we can write Line

B

’s point-slope form equation:

\begin{aligned} y & = m (x - x_{0}) + y_{0} \\ y & = \frac{3}{2} (x - 4) - 3 \end{aligned}

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Reading Questions 3.8.4 Reading Questions

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1.

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Explain the difference between a line that has no slope and a line that has slope

0 .

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2.

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If you make a table of

x

- and

y

-values for a horizontal line what special thing will happen in one of the two columns?

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3.

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If you know two points on one line, and you know two points on a second line, what could you do to determine whether or not the two lines are perpendicular?

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