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# Graphs Of Inequalities

## POINTS ON THE PLANE

❶In the top line x we will place numbers that we have chosen for x.

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Since an equation in two variables gives a graph on the plane, it seems reasonable to assume that an inequality in two variables would graph as some portion or region of the plane. This is in fact the case. To summarize, the following ordered pairs give a true statement. The following ordered pairs give a false statement.

If one point of a half-plane is in the solution set of a linear inequality, then all points in that half-plane are in the solution set.

This gives us a convenient method for graphing linear inequalities. To graph a linear inequality 1. Replace the inequality symbol with an equal sign and graph the resulting line. Check one point that is obviously in a particular half-plane of that line to see if it is in the solution set of the inequality.

If the point chosen is in the solution set, then that entire half-plane is the solution set. If the point chosen is not in the solution set, then the other half-plane is the solution set. Why do we need to check only one point? The point 0,0 is not in the solution set, therefore the half-plane containing 0,0 is not the solution set. Since the line itself is not a part of the solution, it is shown as a dashed line and the half-plane is shaded to show the solution set. The solution set is the half-plane above and to the right of the line.

Since the point 0,0 is not in the solution set, the half-plane containing 0,0 is not in the set. Hence, the solution is the other half-plane. Therefore, draw a solid line to show that it is part of the graph. The solution set is the line and the half-plane below and to the right of the line. Next check a point not on the line. Notice that the graph of the line contains the point 0,0 , so we cannot use it as a checkpoint.

The point - 2,3 is such a point. When the graph of the line goes through the origin, any other point on the x- or y-axis would also be a good choice.

Sketch the graphs of two linear equations on the same coordinate system. Determine the common solution of the two graphs. Example 1 The pair of equations is called a system of linear equations.

We have observed that each of these equations has infinitely many solutions and each will form a straight line when we graph it on the Cartesian coordinate system. We now wish to find solutions to the system. In other words, we want all points x,y that will be on the graph of both equations. Solution We reason in this manner: In this table we let x take on the values 0, 1, and 2. We then find the values for y by using the equation. Do this before going on.

In this table we let y take on the values 2, 3, and 6. We then find x by using the equation. Check these values also.

The two lines intersect at the point 3,4. Note that the point of intersection appears to be 3,4. We must now check the point 3,4 in both equations to see that it is a solution to the system. As a check we substitute the ordered pair 3,4 in each equation to see if we get a true statement.

Are there any other points that would satisfy both equations? Not all pairs of equations will give a unique solution, as in this example. There are, in fact, three possibilities and you should be aware of them.

Since we are dealing with equations that graph as straight lines, we can examine these possibilities by observing graphs. Independent equations The two lines intersect in a single point. In this case there is a unique solution. The example above was a system of independent equations. No matter how far these lines are extended, they will never intersect. Dependent equations The two equations give the same line. In this case any solution of one equation is a solution of the other.

In this case there will be infinitely many common solutions. In later algebra courses, methods of recognizing inconsistent and dependent equations will be learned. However, at this level we will deal only with independent equations.

You can then expect that all problems given in this chapter will have unique solutions. This means the graphs of all systems in this chapter will intersect in a single point.

To solve a system of two linear equations by graphing 1. Make a table of values and sketch the graph of each equation on the same coordinate system. Find the values of x,y that name the point of intersection of the lines. Check this point x,y in both equations. Again, in this table wc arbitrarily selected the values of x to be - 2, 0, and 5.

Here we selected values for x to be 2, 4, and 6. You could have chosen any values you wanted. We say "apparent" because we have not yet checked the ordered pair in both equations.

Once it checks it is then definitely the solution. Graph two or more linear inequalities on the same set of coordinate axes. Determine the region of the plane that is the solution of the system. Later studies in mathematics will include the topic of linear programming. Even though the topic itself is beyond the scope of this text, one technique used in linear programming is well within your reach-the graphing of systems of linear inequalities-and we will discuss it here.

You found in the previous section that the solution to a system of linear equations is the intersection of the solutions to each of the equations. In the same manner the solution to a system of linear inequalities is the intersection of the half-planes and perhaps lines that are solutions to each individual linear inequality. To graph the solution to this system we graph each linear inequality on the same set of coordinate axes and indicate the intersection of the two solution sets.

Note that the solution to a system of linear inequalities will be a collection of points. Again, use either a table of values or the slope-intercept form of the equation to graph the lines. The intersection of the two solution sets is that region of the plane in which the two screens intersect. This region is shown in the graph. Note again that the solution does not include the lines.

In section we solved a system of two equations with two unknowns by graphing. The graphical method is very useful, but it would not be practical if the solutions were fractions. The actual point of intersection could be very difficult to determine. There are algebraic methods of solving systems. In this section we will discuss the method of substitution. Example 1 Solve by the substitution method: Solution Step 1 We must solve for one unknown in one equation.

We can choose either x or y in either the first or second equation. Our choice can be based on obtaining the simplest expression. Look at both equations and see if either of them has a variable with a coefficient of one. Step 2 Substitute the value of x into the other equation. Step 3 Solve for the unknown. Remember, first remove parentheses. Since we have already solved the second equation for x in terms of y, we may use it. Thus, we have the solution 2, In this video, Dr. Carleen Eaton shows you how to graph an inequality with two variables.

This video includes sample problems and step-by-step explanations of systems of equations and inequalities for the California Standards Test. This video includes sample problems and step-by-step explanations of graphing inequalities and testing assertions for the California Standards Test.

Excellent site showing examples of algebra, trig, calculus, differential equations, and linear algebra. Paul's Online Math Notes. This page lays out a detailed explanation on working with inequalities from the beginning. It also provides examples for students to work through and a list and explanation of the different theorems related to inequalities.

Rules for solving inequalities. Again, select any point above the graph line to make sure that it will satisfy or reveal a TRUE statement in terms of the original inequality. Since that point was above our line, it should be shaded, which verifies our solution. A system of inequalities has more than one inequality statement that must be satisfied. Graphically, it means we need to do what we just did -- plot the line represented by each inequality -- and then find the region of the graph that is true for BOTH inequalities.

For the two examples above, we can combine both graphs and plot the area shared by the two inequalities. What is the solution set? We first need to review the symbols for inequalities: Since y is less than a particular value on the low-side of the axis, we will shade the region below the line to indicate that the inequality is true for all points below the line:

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