## RWM102 Study Guide

### Unit 5: Graphs of Linear Equations and Inequalities

### 5a. Graph points with given coordinates on the rectangular coordinate plane

The **coordinate plane** is made up of a horizontal axis, the **x****-axis**, and a vertical axis, the **y****-axis**. When plotting an point on the coordinate plane, you begin at the **origin**, and move left or right based on the value of . Then, from there, move vertically based on the value of . That is the location of your **point**.

For example, let's plot the point

As you can see, we went 3 to the right, because the value is positive three, and then up 7, since the value is positive 7. P is located on the point .

To review, see Points in the Coordinate Plane.

### 5b. Determine coordinates of a point on the rectangular coordinate system

- How do you determine the coordinates of a point on the coordinate plane?

If we see a point on the coordinate plane, we can identify its **coordinates **in the reverse way from how we plotted the point.

Let's find the coordinates of the point . First, consider the -coordinate of the point. Since is 3 to the left, it has an -coordinate of -3. Similarly, has a -coordinate of -3. Therefore, the coordinates of are (-3,-3).

To review, see Points in the Coordinate Plane.

### 5c. Determine whether a given ordered pair is a solution of the equation with two variable

- How can you check if a certain point is the solution to an equation?

When we graph an equation, every point on the graph is a solution to the equation that was graphed. Because of that, we can check if a certain point is a solution to the equation by simply checking if that point is on the graph.

For example, consider the equation . Is the point a solution to the equation?

The graph is:

Since we have been given the graph, all we need to do is check if the point is on the graph. If we move to 3 on the positive \(x)-axis, and then up 1, we find a point that is on the graph. Therefore, is a solution to the equation .

To review, see Ordered Pair Solutions to Equations.

### 5d. Find and graph solutions of the equation in two variables

- How do you find and graph the solution to an equation?

When you have an equation you want to graph the solution of, you should start by finding some specific solutions using an** x-y table**. Then plot those points on the coordinate plane, and finally connect the points to draw the graph.

For example, to graph the solutions to the equation , we will make an table, and select some -values which we will substitute into the equation to find the corresponding -values. First, let's set up the table. You can select any -values you want, but values near the middle of your graph are generally good.

Now we will substitute those -values in for in the equation to find the -values.

Now we have 4 points on our graph. Plot those points, then connect them to graph the equation.

We now have the graph of the solutions to the equation.

To review, see Graphing Linear Equations with Two Variables.

### 5e. Graph a straight line given either its equation, or a slope and y-intercept

- What is slope?
- How do you find and use slope when graphing?

When graphing a linear equation, a key point to focus on is the slope. The **slope **is the change in divided by the change in . We often use the letter "" to represent slope. The slope formula is:

When graphing, the slope of a line can be seen and calculated visually as well.

To calculate the slope visually, simply identify two points on the line, then count the change in y and change in x between those points, sometimes called "**rise over run**". Be sure to be careful to consider if the points are changing positively (up/right) or negatively (down/left) to accurately calculate the slope.

For example, we will calculate the slope of the following line:

If we focus on the points (-5,1) and (0,3), we can see that between these points, the y went up 2, and the went to the right 5. Therefore our slope is .

When graphing a line, you can use any point along with the slope to make your graph.

For example, let's graph a line passing through the point (-3, 1) with a slope of ⅔.

First, we will plot a point at (-3,1). Then from that point, we will move according to the slope, ⅔. We will move up 2 and to the right 3, and arrive at another point on the line, the point (0,3). Finally, connect these points and you will have the graph of your line.

To review, see Understanding the Slope of a Line.

### 5f. Find slope and intercepts of a straight line given its equation or its graph

- How do you find the -intercept of a line?
- How do you find the -intercept of a line?
- How do you graph a line in slope-intercept form?

When graphing a line, one easy way to find some important points is to find the **x-intercept** and **y-intercept.** When viewing a graph, the intercepts can be found by simply looking where the line crosses the -axis and -axis.

For example, the linear function above has a -intercept of (0,-3) and an -intercept of (2,0).

If you have the equation of a line, finding the intercepts is quite simple. To find the -intercept, you simply let the , and solve for . Similarly, to find the -intercept, let and solve for .

For example, to find the intercepts of , first we will let to find the -intercept. , , . Therefore the -intercept is (0,-3). Similarly, we can find the -intercept by letting . , , . Therefore the -intercept is (2,0).

One way the equation of a line can be written is called slope-intercept form. **Slope-intercept form** is , where the is the slope, and is the -intercept. In slope-intercept form, we can start by plotting the -intercept, then use the slope to find another point and graph the line.

For example, the line , has a -intercept of (0,-3) and a slope of 2. To graph, we begin by plotting the y-intercept, then from that point, graphing a slope of 2 to find another point and draw the graph.

To review, see Graphs with Intercepts and Using the Slope-Intercept Form of an Equation of a Line.

### 5g. Write the equation of a line passing through two given points

- How do you write the equation of a line passing through two points?

Another way to write the equation of a line is called point-slope form. **Point-slope form** is . In point-slope form, is a point on the line, and is still the slope. This form is ideal for problems where you are asked to write the equation of a line, as you only need any point on the line and the slope to write the equation.

For example, to find the equation of the line passing through (-2,3) and (-1,-2), first we must find the slope. Using the slope equation, the slope is . Now that we have the slope, we can use either point to write the equation of the line. If we use the point (-2,3), then the equation of the line is .

To review, see Linear Equations in Point-Slope Form.

### 5h. Write the equation of a line with a given slope passing through a given point

- How do you write the equation of a line given a slope and a point?

When a slope and a point are given, rather than two points, writing the equation of a line is even simpler with point-slope form. Since a point and the slope are all that are needed to write the equation, you simply need to plug in the information given.

For example, to find the equation of the line passing through (-2,5) with a slope of ⅓, simply substitute into the point-slope equation,.

To review, see Linear Equations in Point-Slope Form.

### 5i. Locate on a coordinate plane all solutions of a given inequality in two variables

- How do you graph the solutions to a linear inequality?

**Linear inequalities** are very similar to linear equations, except instead of just finding solutions on the line, we will be finding an entire area of the graph that has solutions to our inequality.

To graph a linear inequality, such as , start by graphing the **equivalent equation**, . The y-intercept is (0,-1) and the slope is 3. When graphing, draw a dashed line, instead of a solid line. For inequalities with the < or > symbols, always use a dashed line. For inequalities with the or symbols, you can use a solid line.

Now, pick any point on one side of the line. We will test that point in our inequality to see if it satisfies the inequality. If it does, then we will shade that side. If it doesn't, then we will shade the other side. For example, we will test the point (0,0), which is on the left/upper side of the line. simplifies to . This is not true. Therefore we must shade the other side.

To review, see Graphs of Linear Inequalities.

### 5j. Represent relationships between quantities as an equation or inequality in two variables

- How do you represent the relationship between quantities in an inequality?

Inequalities are used every day in our lives. For example, if you want to buy gas and snacks, but only have $20, you have solved an inequality. For example, if gas is $3 per gallon, and snacks are $4 each, you can create an inequality such as . You can test values for g and s to find possible solutions, such as , . Since , and , then , is a possible solution.

To review, see Graphs of Linear Inequalities.

### 5k. Interpret the meaning of slope and intercepts of the graph of a relationship between quantities

- How can you identify parallel lines from their slopes?
- How can you identify perpendicular lines from their slopes?

**Parallel lines** are two lines that never intersect. Think of parallel lines like the lines on a highway, they never intersect. When looking at the equations of two lines, the key to determining if the lines are parallel is to examine their slopes. Parallel lines must have the same slope. For example, the lines and are parallel because they both have a slope of 2.

**Perpendicular lines** are two lines that intersect at a 90 degree angle. The **product** of the slopes of two perpendicular lines is always equal to -1. Another way to identify perpendicular lines is that the slope of one line is the **opposite reciprocal** of the other line. The opposite means change the sign, and reciprocal means to flip the number, making the numerator the denominator, and vice versa. For example, the lines and are perpendicular since the opposite reciprocal of 2 is .

To review, see Parallel and Perpendicular Lines.

### Unit 5 Vocabulary

This vocabulary list includes terms listed above that students need to know to successfully complete the final exam for the course.

- coordinate plane
- x-axis
- y-axis
- origin
- point
- coordinates
- x-y table
- slope
- rise over run
- x-intercept
- y-intercept
- slope-intercept form
- point-slope form
- linear inequalities
- equivalent equation
- parallel lines
- perpendicular lines
- product
- opposite reciprocal