9.6: Linear Functions
Introduction
Banana Math
“The vegetation of the rainforest was very interesting,” Carmen commented in Mr. Thomas’ class as the rainforest discussion continued.
“There were so many different things growing,” Mark agreed.
The students began having a discussion about the things that had intrigued them about the plant life of the rainforest. One of the points that was brought up is that there are plants in the rainforest that can’t be found anywhere else in the world. Mr. Thomas spotted the opportunity and wrote the following problem.
You buy a banana tree that is 8 inches tall. It grows 4 inches per day. Its height (in inches)
You can express this function as an equation and a graph. That is the work that you will learn about in this lesson. Pay close attention and you will be ready to complete both of these parts by the end of the lesson.
What You Will Learn
In this lesson, you will learn how to complete the following skills.
 Write linear equations given two points on a line,
x andy – intercepts, slope andy – intercept, or a table of values.  Use function notation to express a linear relationship between an independent and dependent variable.
 Graph functions given in function notation.
 Model and solve real  world problems using tables, equations and graphs of linear functions.
Teaching Time
I. Write Linear Equations Given Two Points on a Line,
The
Example
We know that we are going to use the form of the equation
This is the answer. The key is to always watch for negative signs and be sure to include them when you write your equation.
Sometimes, we can also be given the slope and a point that the line crosses through. We can also use this information to write an equation of a line. Let’s look at an example.
Example
With this example, we know the slope, so that can be easily substituted into the slopeintercept form. The point has a 0 for the
This is the answer.
What if you only know two points and you don’t know the slope? It’s a similar operation that we can use in order to write the equation.
Do you recall that the slope formula is
We will use the notion below.
if
Example
Write the equation of the line that passes through the points
Now plug in our known values of
Do you see that we have a proportion? This can be solved by cross multiplying.
This is the answer.
What about when you have been given a table of values? Well, there is a way to figure out the equation quite simply when you have a table of values. Take a look at the example.
Example



0  5 
1  7 
2  9 
3  11 
4  13 
First, notice that the
Now we need to figure out the slope. Look at the y values in the table. Can you see a pattern? If you look carefully, you will see that the values jump by +2 each time. This is the slope. Think about how the line would move when graphed. The pattern of the
This is the answer.
II. Use Function Notation to Express a Linear Relationship Between an Independent and Dependent Variable
In science, an independent variable is a parameter that is manipulated or chosen by a scientist while the dependent variable is a parameter that is measured. Scientists oftentimes look for a correlation between an independent variable and a dependent variable—they want to know if the dependent variable depends on the independent variable. For example, a scientist might measure the speed at which a car is moving and the force upon impact when the car hits a wall. The scientist can manipulate the speed of the car—she can make the car move slower or faster. She would then measure the force of impact related to the given speed. Then, a conclusion can be drawn about their relatedness and cars, in this case, might be designed based on that relation. The independent variable will be shown in the left column of a ttable and on the
If you think about this it makes perfect sense. We have talked about how the function of \begin{align*}y\end{align*}
\begin{align*}f(x)=4x+1\end{align*}
Here we know that the function of \begin{align*}x\end{align*}
Once you understand the connection between independent variables and dependent variables, we can move on to graphing these linear equations.
III. Graph Functions Given in Function Notation
As with any other linear equation, functions can be graphed using a ttable or using the slopeintercept form. It is important to place the independent variable on the \begin{align*}x\end{align*}
Example
You buy a orange tree that is 12 inches tall. It grows 3 inches per day. Its height (in inches) \begin{align*}h\end{align*}
\begin{align*}h=3d+12\end{align*}
Let’s use slopeintercept form to show its graph. We know that the slope is 3 and the \begin{align*}y\end{align*}
IV. Model and Solve Real – World Problems Using Tables, Equations and Graphs of Linear Functions
There are so many ways that we can use functions in real life. Think about all of the things that you do in a day. If you drive to work, then the time that it takes you to drive is a function of the speed of your car. If you are paid hourly, then the amount of money that you make is a function of the number of hours that you work. Each and every day we use functions. Let’s take a look at another example.
Example
A group of students measure the length of their classmates’ arms and legs and found the following data:
arm(in)  legs(in) 

25  30 
27  33.2 
26  31.6 
Assume there is a linear relationship. Write and graph the linear function that describes this data.
Use the table to determine ordered pairs. Then you can find the slope. Find the slope using the slope formula for \begin{align*}x_1=25, y_1 = 30, x_2 = 27, y_2 = 33.2\end{align*}
\begin{align*}m &=\frac{{y_2}{y_1}}{{x_2}{x_1}}\\
m &=\frac{33.230}{2725}\\
m &=\frac{3.2}{2}\\
m &=1.6 \\
m &=\frac{8}{5}\end{align*}
Now substitute in our known values of \begin{align*}m, x_1,\end{align*}
\begin{align*}m &=\frac{{y}{y_1}}{{x}{x_1}}\\
\frac{8}{5} &=\frac{y30}{x25}\\
5(y30) &=8(x25)\\
5y150 &=8x200 \\
5 &=8x50\\
y &=\frac{8}{5}x10\end{align*}
Now we can graph the data.
Now let’s go back to the problem from the introduction and use what we have learned.
RealLife Example Completed
Banana Math
Here is the original problem once again. Reread it and then complete both parts of the solution. Write an equation and draw a graph for this problem.
“The vegetation of the rainforest was very interesting,” Carmen commented in Mr. Thomas’ class as the rainforest discussion continued.
“There were so many different things growing,” Mark agreed.
The students began having a discussion about the things that had intrigued them about the plant life of the rainforest. One of the points that was brought up is that there are plants in the rainforest that can’t be found anywhere else in the world. Mr. Thomas spotted the opportunity and wrote the following problem.
You buy a banana tree that is 8 inches tall. It grows 4 inches per day. Its height (in inches) \begin{align*}h\end{align*}
Remember there are two parts to this solution.
Solution to Real – Life Example
First, we need to write an equation. We can use the \begin{align*}h\end{align*}
\begin{align*}h=4d+8\end{align*}
Let’s use slopeintercept form to show its graph.
\begin{align*}m=4,b=8\end{align*}
Notice that we only need the first quadrant of the coordinate plane because negative values would not make sense.
Vocabulary
Here are the vocabulary words in this lesson.
 Independent Variable

a value that is not dependent on another value. It is the \begin{align*}x\end{align*}
x value in a table.
 Dependent Variable

a value that is dependent on the equation. It is the \begin{align*}y\end{align*}
y value in a table.
 Function Notation

an equation where the value of \begin{align*}x\end{align*}
x is dependent on the equation involving \begin{align*}x\end{align*}x .
Time to Practice
Directions: Write the equation of a line with the following slopes and \begin{align*}y\end{align*}

\begin{align*}\text{slope} = 2, \ y \ \text{int} = 4\end{align*}
slope=2, y int=4 
\begin{align*}\text{slope} = 3, \ y \ \text{int} = 2\end{align*}
slope=−3, y int=2 
\begin{align*}\text{slope} = 4, \ y \ \text{int} = 4\end{align*}
slope=−4, y int=4 
\begin{align*}\text{slope} = 3, \ y \ \text{int} = 5\end{align*}
slope=3, y int=−5 
\begin{align*}\text{slope} = \frac{1}{2}, \ y \ \text{int} = 2\end{align*}
slope=12, y int=−2 
\begin{align*}\text{slope} = \frac{1}{3}, \ y \ \text{int} = 2\end{align*}
slope=−13, y int=2 
\begin{align*}\text{slope} = 1, \ y \ \text{int} = 8\end{align*}
slope=1, y int=8 
\begin{align*}\text{slope} = 2, \ y \ \text{int} = 4\end{align*}
slope=−2, y int=4 
\begin{align*}\text{slope} = 1, \ y \ \text{int} = 1\end{align*}
slope=−1, y int=−1 
\begin{align*}\text{slope} = 5, \ y \ \text{int} = 2\end{align*}
slope=5, y int=−2
Directions: Write the following horizontal or vertical line equations.
 A horizontal line with a \begin{align*}b\end{align*}
b value of 7.  A horizontal line with a \begin{align*}b\end{align*}
b value of 4.  A vertical line with an \begin{align*}x\end{align*}
x value of 2  A vertical line with an \begin{align*}x\end{align*}
x value of 5
Directions: Write the equation of a line that passes through the following points.
 (3, 3) and (3, 1)
 (2, 3) and (0, 3)
 (2, 3) and (3, 4)
 (2, 3) and (3, 4)
Directions: Use what you have learned to solve each problem.
 A migrating monarch butterfly travels 1100 miles. If it flies 30 miles per day, the distance \begin{align*}d\end{align*}
d it still has to travel is a function of days \begin{align*}t\end{align*}t it has traveled. Write a function rule for the situation.  A writer gets paid a writer’s fee of $3000 plus $1.50 for each copy of the book that is sold. Create a function rule for this situation.
 How many books does the writer need to sell to earn $10,000 total?
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