**Learning Objectives**

After completing this tutorial, you should be able to:

- Find the
*x*- and*y*-intercepts of a linear function. - Graph a linear function using the
*x*- and*y*-intercepts. - Graph vertical and horizontal lines.

** Introduction**

In **Tutorial 21:
Graphing Linear
Equations**, we went over graphing linear equations by plotting
points.
In this tutorial the concept of using intercepts to help graph will be
introduced, as well as vertical and horizontal lines. Actually,
the
process of graphing by plotting points and graphing by using
intercepts
are essentially the same. Intercepts are just special types of
solutions,
but solutions none the less. So once we find them, we plot them
just
the same as any other ordered pair that is a solution. Once we
plot
them, we draw our graph in the same fashion as when we had
non-intercept
points. So, basically, when you graph, you plot solutions
(whether
they are intercept points or not) and connect the dots to get your
graph.
Just keep in mind that intercepts work with the number 0, which is a
nice
easy number to work with when plugging in and solving. (**It is
one of those types of problems that I warn students to not make harder
than it is**).

** Tutorial**

The word 'intercept' looks like the word
'intersect'. Think
of it as **where the graph intersects the x-axis**.

With that in mind, **what
value is y always going to be on the x-intercept?**
No matter where you are on the

If the* x*-intercept is
where the graph crosses
the *x*-axis where do you think **the
graph
crosses for the ***y*-intercept?
If you
said the *y*-axis, you are
absolutely
right.

This time** it is x’s
value that is 0**. Any where you would cross the

Below is an illustration of a graph of a linear function
which** highlights
the x and y intercepts:**

In the above illustration, the ** x-intercept
is the point (2, 0)** and the

**Keep in mind that the x- and y- intercepts are two separate
points. There
is only one point that can be both an x- and y- intercept** at the same time, do you know what point that is?

If you said the **origin (0, 0)**, give yourself a
pat on the back.

You find the *x*-intercept
by plugging in
0 for* y* and solving for *x.*

You find the *y-*intercept
by plugging in
0 for *x* and solving for *y*.

You do this by plugging in ANY value(s) for *x* and finding it's corresponding *y* value.

This is just like we showed you in **Tutorial
21: Graphing Linear Equations.**

Remember that intercepts are points on the graph,
too. They are
plotted just like any other point.

If you need a review on plotting points go to **Tutorial
20: The Rectangular Coordinate System.**

The graph of a linear function is a straight line.

What value are we going to use for

You are correct if you said

***Inverse of mult. by -3 is div.
by -3**

**Next, we will find the y-
intercept.**

What value are we going to plug in for *x*?

If you said *x *= 0 you are right.

We can plug in any *x *value
we want as long
as we get the right corresponding *y* value and
the function exists there.

**Let’s put in an easy number x =
1:**

So **the ordered pair (1, 2) is another solution** to our function.

Note that we could have plugged in any value for x: 5,
10, -25, ...,
but it is best to keep it as simple as possible.

**The solutions that we found are:**

What value are we going to use for

You are correct if you said

**Next, we will find the y-
intercept.**

What value are we going to plug in for

If you said

Hey, look at that, we ended up with the exact same point
for both our *x*-
and *y-*intercepts. As mentioned
above,
there is only one point that can be both an* x*-
and *y*- intercept at the same time, the
origin
(0, 0).

We can plug in any* x *value
we want as long
as we get the right corresponding *y *value
and
the function exists there.

**Let’s put in an easy number x = 1:**

So **the ordered pair (1, -3) is another solution** to our function.

**Let’s put in another easy number x = -1:**

So **the ordered pair (-1, 3) is another solution** to our function.

Note that we could have plugged in any value for x: 5,
10, -25, ...,
but it is best to keep it as simple as possible.

**The solutions that we found are:**

*x *= *c*

If you have an equation* x* = *c*,
where *c* is a constant, and you are
wanting
to graph it on a two dimensional graph, this would be a vertical line
with *x*-intercept
of (*c*, 0).

Even though you do not see a *y *in
the equation,
you can still graph it on a two dimensional graph. Remember that
the graph is the set of all solutions for a given equation. If
all
the points are solutions then any ordered pair that has an *x* value of *c* would be a solution. As
long
as *x *never changes value,
it is
always *c*, then you have a
solution. In
that case, you will end up with a vertical line.

**Below is an illustration of a vertical line x = c:**

*y *= *c*

If you have an equation *y* = *c*,
where *c* is a constant, and you are
wanting
to graph it on a two dimensional graph, this would be a horizontal line
with *y*- intercept of (0, *c*).

Even though you do not see an *x *in
the equation,
you can still graph it on a two dimensional graph. Remember that
the graph is the set of all solutions for a given equation. If
all
the points are solutions then any ordered pair that has an *y* value of *c* would be a solution. As
long
as y never changes value, it is always c, then you have a
solution.
In that case, you will end up with a horizontal line.

**Below is an illustration of a horizontal line y = c:**

It looks like it fits the form* y* = *c.*

With that in mind, what kind of line are we going to end up with?

Horizontal.

**Note how the directions did not specify that we had
to use intercepts
to do our graph. ** **Any time you take a math test or do
homework,
make sure that you follow directions carefully. If it specifies a
certain way to do a problem, then you need to follow that plan (like in
the above examples 1 and 2). If it does not specify, like in this
example, then you can use what ever “legitimate” way works to get the
job
done.**

Since this is a special type of line, I thought I would
talk about
steps 1 and 2 together.

It doesn’t matter what *x* is, *y* is always 4. So for our solutions we just need three ordered
pairs
such that *y* = 4.

**Note that the y-intercept
(where x =
0) is at (0, 4). **

**Do we have a x-intercept? The answer is no**.
Since *y* has to be 4, then it can never equal 0, which is the criteria of an *x*-intercept.
Also, think about it, if we have a horizontal line that crosses the *y-*axis
at 4, it will never ever cross the *x-*axis.

**So, some points that we can use are (0, 4), (1, 4)
and (2, 4).
These are all ordered pairs that fit the criteria of y having to be 4.**

Of course, we could have used other solutions, there are
an infinite
number of them.

**The solutions that we found are:**

Note how if we subtract 3 from both sides, we can write
this as *x* = -3, which means it can be written in the form *x* = *c*.

So, what type of line are we going to end up with?

Vertical.

So, what type of line are we going to end up with?

Vertical.

Since this is a special type of line, I thought I would
talk about
steps 1 and 2 together.

It does not matter what *y* is, as long as *x* is -3.

**Note that the x-intercept
is at (-3,
0).**

**Do we have a y-intercept?**

**Some points that would be solutions are (-3, 0), (-3,
1), and (-3,
2).**

Again, I could have picked an infinite number of
solutions.

**The solutions that we found are:**

** Practice Problems**

These are practice problems to help bring you to the
next level.
It will allow you to check and see if you have an understanding of
these
types of problems. **Math works just like
anything
else, if you want to get good at it, then you need to practice
it.
Even the best athletes and musicians had help along the way and lots of
practice, practice, practice, to get good at their sport or instrument.**
In fact there is no such thing as too much practice.

To get the most out of these, **you should work the
problem out on
your own and then check your answer by clicking on the link for the
answer/discussion
for that problem**. At the link you will find the answer
as well as any steps that went into finding that answer.

Practice Problems 1a - 1b:Graph each linear function by findingx- andy-intercepts.

Practice Problems 2a - 2b:Graph each linear equation.

2a. *x* = 4

(answer/discussion
to 2a)

** Need Extra Help on these Topics?**

**The following is a webpage
that can assist
you in the topics that were covered on this page: **

**http://www.purplemath.com/modules/intrcept.htm**

This webpage goes over *x*- and *y*-intercepts.

**Go to Get
Help Outside the
Classroom found in Tutorial 1: How to Succeed in a Math Class for
some
more suggestions.**

Last revised on July 31, 2011 by Kim Seward.

All contents copyright (C) 2001 - 2011, WTAMU and Kim Seward. All rights reserved.