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Graphs Using Slope-Intercept Form

Use the y-intercept and the 'rise over run' to graph a line

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Graphing Linear Equations

Let’s Think About It

Credit: Zach Dischner
Source: https://www.flickr.com/photos/zachd1_618/8042036522/in/photolist-dfDxh7-cy6tUC-a9nq86-6TGSGJ-bgQ7AD-8tSwSX-8tSxS8-bywhTi-6t1SRk-tBzBKQ-9gzb95-4fs4BB-btpvcq-77sXEc-ciNhEU-4A7cw1-amsK7q-4zpMEQ-8gsWjq-69mgoQ-79YxUy-rKTLhr-fkKUHe-qmVcco-tnBYM9-KZt3M-8oWGEe-8vGnwi-btpts5-cbDLwu-bGjhhB-oYA9k3-qrnPn9-srAss-7mWdSw-bN76En-pv7N6r-fFHppn-pq5SnD-qj4kzg-9CaMyE-7mSkfi-8pCtJx-5P4kXc-bUhy1v-9jH585-aPe6YF-7uv953-8pFCNE-z2UrJ
License: CC BY-NC 3.0

The local hiking club is finalizing its plans for a trip to the Rocky Mountains in Canada. The hikers are discussing the food they will have to carry with them to complete their climb up the mountains. The group decides on setting out with 84 pounds of food and figure they will eat 11 pounds each day. To make sure they will have enough food, the members of the club want to show how their food supply looks at the end of each day as well as at the end of their trip. What can the hikers do to create a visual picture of their food supply?

In this concept, you will learn to recognize functions

Guidance

When graphing a straight line, two points on the line must be plotted and joined. If an equation is written in standard form, the - and -intercepts can be calculated and plotted on the Cartesian grid. These plotted points can then be joined to graph the line represented by the equation.

Let’s look at an example.

Plot the graph of the line represented by the following equation: 

Look at the given equation. It is written in standard form.

 such that  and .

First, calculate the -intercept using the formula: 

Next, fill in the values for  and

The answer is 5.

The -intercept is .

Second, calculate the -intercept using the formula: 

Next, fill in the values for  and .

The answer is 3.

The -intercept is .

Next, plot the points  and  on the Cartesian grid and join them with a straight line.

License: CC BY-NC 3.0

However, the equation of a line is not always written in standard form. The equation of a straight line is often written in slope intercept form. Remember, the slope-intercept form of an equation is written as:

 such that ‘’ is the slope of the line and ‘’ is the -intercept of the line. These two values can be used to plot two points on the Cartesian grid. Remember the value of the slope should be expressed in fraction form for graphing. The denominator of the fraction  is  and should always be a positive value. A positive value means that the movement in a horizontal direction will always be to the right. The numerator of the fraction is  and can be either a negative or a positive value.

Let’s look at an example.

Plot the graph of the line represented by the following equation: 

Look at the given equation. It is written in slope-intercept form.

 such that  and  or .

First, plot the -intercept  on the Cartesian grid. From here, count to the right the bottom of the slope (3) and then up the top of the slope (2).

License: CC BY-NC 3.0

Then, join the two plotted points with a straight line.

License: CC BY-NC 3.0

Guided Practice

Plot the graph of the following equation written in slope-intercept form. 

First, write down the information given in the equation: 

 such that  and .

Next, plot the -intercept  on the Cartesian grid. From the -intercept move to the right 4 units, move downward 3 units, and plot the point.

Then, join the two plotted points with a straight line.

License: CC BY-NC 3.0

Examples

Example 1

Plot the graph of the line represented by the following equation: 

Look at the given equation. It is written in standard form.

 such that  and .

First, calculate the -intercept using the formula: 

Next, fill in the values for  and .

The answer is -6.

The -intercept is .

Second, calculate the -intercept using the formula:

Next, fill in the values for  and .

 The answer is 4.

The -intercept is .

Next, plot the points  and  on the Cartesian grid and join them with a straight line.

License: CC BY-NC 3.0

Example 2

Plot the graph of the following equation written in slope-intercept form. 

First, write down the information given in the equation:

 such that  and .

Next, plot the -intercept  on the Cartesian grid. From the -intercept move to the right 5 units, move upward 2 units, and plot the point.

Then, join the two plotted points with a straight line.

License: CC BY-NC 3.0
 

Example 3

Rewrite the following equation written in standard form in slope-intercept form. State the slope and -intercept of the line. 

First, subtract  from both sides of the equation and simplify.

Next, divide both sides of the equation by  to solve for .

Then, write the equation in slope-intercept form.

The slope of the line is  and the -intercept is .

Follow Up

Credit: Richard Allaway
Source: https://www.flickr.com/photos/geographyalltheway_photos/6658220667/in/photolist-b9n834-8pFCxU-8tVB49-4pdAgG-qrwp7X-8tVBkm-8tVBBh-jnuyMb-8pCu86-8pCsec-8pCuct-8pFDPL-btpw2N-fY7hZC-8tSxma-8tVBZw-eTd9Ry-eTd8Dm-7BXnV5-8z7kCi-6eNWuM-9CbXDs-6RP261-oJteiA-6CR2Xa-8tVB7j-eTJimP-eSwcxE-eTd9cs-eT1Tvn-eT1Lhr-eTd5WA-eT1V6H-eT1QcK-eTdbYY-dySqFo-2EPLZU-8wGPRW-e2E72o-5PuFPX-8tVBo5-qxpk8E-io4t41-5gEVAN-eT1SZT-eT1LLx-eTJQGp-eTWbLd-eSiGuX-eSjfmV
License: CC BY-NC 3.0

Remember the hiking club and the food? The hikers need to keep track of the 84 pounds of food they are carrying. If they eat 11 pounds of food a day, how can they create a visual display of the food they have left at the end of each day?

First, write down the given information

 pounds of food left at the end of each day

 pound of food eaten each day

 number of days hiking

 number of pounds of food being carried

Next, substitute the known values into the equation.

Then, graph the equation on the Cartesian grid.

License: CC BY-NC 3.0

Video Review

https://www.youtube.com/watch?v=uk7gS3cZVp4

Explore More

Use what you have learned to complete each task.

1. Write this equation in slope-intercept form.

2. What is the slope?

3. What is the -intercept?

4. Graph the equation. 

5. Write the following equation in slope-intercept form.

6. What is the slope?

7. What is the -intercept?

8. Graph the equation.

Use what you have learned to solve each problem.

Miguel wants to save $47 for a video game. He received $20 as a gift and gets $4 per week for allowance.

9. Write an equation in slope-intercept form that represents this situation.

10. How long will it take him to save enough money?

11. What is the slope of this line?

12. Which form is the equation written in?

13. What is the -intercept of this line?

14. What is the graph of this line?

15. Is this a linear graph? How can you tell?

Vocabulary

y-intercept

y-intercept

A y-intercept is a location where a graph crosses the y-axis. As a coordinate pair, this point will always have the form (0, y).
Cartesian Plane

Cartesian Plane

The Cartesian plane is a grid formed by a horizontal number line and a vertical number line that cross at the (0, 0) point, called the origin.
linear equation

linear equation

A linear equation is an equation between two variables that produces a straight line when graphed.
Linear Function

Linear Function

A linear function is a relation between two variables that produces a straight line when graphed.
Slope

Slope

Slope is a measure of the steepness of a line. A line can have positive, negative, zero (horizontal), or undefined (vertical) slope. The slope of a line can be found by calculating “rise over run” or “the change in the y over the change in the x.” The symbol for slope is m
Slope-Intercept Form

Slope-Intercept Form

The slope-intercept form of a line is y = mx + b, where m is the slope and b is the y-intercept.

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