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Linear Relationships

Solving Linear Systems


System of Linear Equations

A system of linear equations is a set of two or more linear equations. To show that equations are part of the same system, they are usually written on top of each other and have a curly bracket to the left. It's not unusual to add Roman numerals, to be able to refer to the equations individually. Systems of linear equations often contain more than one unknown variable where the solution is the set of coordinates that make all equations true simultaneously. In the example above, the solution is and These coordinates make the sides equal in both equations. The solution is usually written as a point:

A system of linear equations can be solved both graphically and algebraically by using the substitution method or elimination method.


Solving a System of Linear Equations Graphically

To solve a system of linear equations graphically means graphing the lines and identifying the point of intersection.

For example, the following system, can be solved by graphing.


Write the equations in slope-intercept form
Start by writing the equations in slope-intercept form by solving for For the first equation, divide both sides by The second equation can be rewritten by adding on both sides and rearranging it.
The second equation, written in slope-intercept form, is meaning that the system can be written as


Graph the lines

Next, graph the lines on the same coordinate plane. Here, the -intercepts are and the slopes are and


Identify the point of intersection

The point where the lines intersect is the solution to the system.

The lines appear to intersect at Thus, this is the solution to the system.

Note that whenever a system of equations is solved graphically, the solution is approximate.

In a football game, the home team, the Mortal Wombats, defeated the Fearless Seagulls by points. The total score for both teams was What was the final score?

Show Solution

To begin, we'll use variables to represent the different quantities. Let be the number of points the Wombats scored and be the number of points the Seagulls scored. The Wombats scored more points than the Seagulls. Thus, the difference between and can be written as The total amount of points was so the sum of and is Both of these equations must be true simultaneously, giving us the following system of equations. We can solve the system by graphing. First, let's write the second equation in slope-intercept form by subtracting on both sides. Now, we can graph the lines. Since the scores cannot be negative, we only graph the lines for positive values of and

Now, we can identify the point of intersection.

The point of intersection is This means, the Wombats scored points and the Seagulls scored


Substitution Method

This is an algebraic method of finding the solutions to a system of equations. It consists of substituting an equivalent expression for a variable in one of the equations of the system. Consider an example system of linear equations. To solve the system by using the Substitution Method, there are four steps to follow.


Isolate One Variable in Any of the Equations
The first step is to isolate any variable in any of the equations. For simplicity, in this case the variable will be isolated in Equation (I).


Substitute the Expression
In the equation where the variable was not isolated, substitute the obtained expression for the variable. In this case, will be substituted for in Equation (II).
Now, Equation (II) only has one variable, which is


Solve the Equation With One Variable
Solve the equation that contains only one variable. In this case, Equation (II) will be solved for
The value of the variable, in this case, is


Substitute the Value of the Variable in the Other Equation
Since the value of one variable is known, it can be substituted in the equation that has not been considered yet. In this case, will be substituted in Equation (I).
Evaluate right-hand side
The value of the variable, in this case, is Therefore, the solution to the system of equations, which is the point of intersection of the lines, is or
If at any step of the method a true statement is obtained, then the lines represented by the equations of the system are coincidental. Therefore the system has infinitely many solutions. Conversely, if at any step a false statement is obtained, then the lines are parallel. In this case, the system has no solution.

The sum of two numbers is One of the numbers is two more than three times the other number. Write a system that represents the given relationships. Then, find the numbers using substitution.

Show Solution

We can use the given information to write two equations. First, we must define our variables. Let the first number be and the other We know that the sum of these numbers is Thus, We also know that one of the numbers, let's say is two more than three times the other number, which is then This gives us the equation Together these two equations create the system To solve this system using substitution, we must substitute one equation into the other. Let's substitute into This will allow us to then solve for

We've found that the -coordinate of the solution is We can substitute this value into either equation to solve for the corresponding -value. We'll choose the second equation.


Since the solution to the system is


System of Inequalities

A system of inequalities is a set of two or more inequalities that are solved together. The following system of inequalities contains two conditions on the variables and Systems of inequalities are often illustrated graphically in a coordinate plane, where the inequalities define a region.


Graphing a System of Linear Inequalities

Individual inequalities can be interpreted graphically as the area above or below a boundary line. When all inequalitites in a system are graphed, the solution to the system is the overlapping region of the individual solution sets. For example, the following system can be solved in this way.


Write the inequalities in slope-intercept form
To be able to graph the inequalities individually, start by writing them in slope-intercept form.


Graph the inequalities in a coordinate plane

To graph the inequalities, begin with the boundary lines. The inequality has the boundary line Since the inequality sign is the line should be dashed, and the region below the line is shaded.

Similarly, has the boundary line The inequality sign is so the line is dashed and the region below the line is shaded.


Find the overlapping region

Notice that the individual solution sets overlap for a portion of the graph. This overlapping region is the solution set of the system. The points in this region are all the points that satisfy both inequalities. In this case, this is the purple region.

Lastly, since the boundary lines in their entirety are not part of the solution set, trim them to only show the borders of the overlapping region.


Marco's mother asks him to buy burritos and tacos from the restaurant near their house. She gives him and instructs him to get enough food so that they can feed people. If burritos cost each and tacos cost each, how many of each can he buy?

Show Solution
There are a number of conditions in this exercise. Let's start with making sense of them. Let's use to represent the number of burritos that Marco buys, and to represent the number of tacos. Since the burritos cost each, the total amount spent on just burriots is Similarly, since a taco costs the total cost of tacos is Therefore, The total cost can be expressed as Since Marco has he cannot spend more than this. The total must be less than or equal to That means, We also know that Marco needs to buy enough food to feed people. Assuming no one wants to share a taco or a burrito, the total amount of dishes must be at least This gives Since both of these inequalities must hold true, we get the following system of inequalities. Solving this system gives all of the possible combinations of burritos and tacos Marco can purchase while buying enough food and staying within his budget. To solve the system, we must graph it. Let's first write the inequalities in slope-intercept form by isolating
Solve for
The first inequality can be expressed as Writing the second inequality in slope-intercept form can be done in one step. Specifically, by subtracting on both sides. The system can now be expressed as We'll graph each inequality by showing its boundary line and shading the appropriate region.

The purple region represents the solution set that satisfies both inequalities. Since Marco can't buy a negative number of burritos or tacos we're only interested in the positive values of and

Any point in this region corresponds to a combination on burritos and tacos that costs less than and feeds at least people. Let's look at the corners of this region.

The marked points represent minimum and maximum possibilities.

  • and represent Marco buying either tacos or burritos. Then he'd feed exactly people, and have money remaining.
  • and tell how many of each dish Marco can buy if he used all money on and only bought tacos or burritos. Thus, he can buy burritos or tacos.

Since we don't know how hungry the guests are, or what their preferences are, Marco should buy both burritos and tacos. Let's choose a point in the middle of the region.

One possibility is that Marco can purchase tacos and burritos. That way, there will probably be enough food, and he'll have money left. Note that even though decimal numbers are a part of the solution set, the answer should be given in whole numbers, assuming you can't buy a part of a taco or burrito.

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