## 17Calculus Precalculus - Systems of Equations

Types of Systems of Equations

Linear with the same number of equations as unknowns - covered on this page

Linear with fewer equations than unknowns - covered on the dependent system page

Non-Linear - covered on the non-linear system page

Systems of equations are made up of a set of two or more equations that contain multiple variables. Linear systems have all linear terms, i.e. all the variables have a power of one and none of the variables are multiplied together. Nonlinear systems have at least one variable with a power other than one, like a square root or a squared term, or have variables multiplied together.

Linear Systems of Equations - 3 Cases

For example, here is a linear system of two equations and two unknowns, x and y.
$$\begin{array}{rcrcl} 2x & + & y & = & 3 \\ x & - & 3y & = & 0 \end{array}$$
Notice that we have two variables and two equations. In this case, we have three possible types of solutions.
1. We have one, unique solution. If we think of these equations as lines in the plane, there is one point where they intersect.
2. We have no solution. In this case, the lines would be parallel and never intersect.
3. We have an infinite number of solutions, i.e. the lines completely overlap and the two equations represent the same line. When we have an infinite number of solutions or we have fewer equations than variables, then we have what is called a dependent system of equations. We can get some equations that 'solve' the system, i.e. we can get a set of equations that represent the infinite set of solutions. We cover that idea on a separate page. On this page, we will stick with linear systems where we have the same number of equations as variables.

Before we go on, let's watch a quick video clip about some terminology related to these three cases of linear systems.

### Thinkwell - Three Cases for Linear Systems [1min-10secs]

video by Thinkwell

Graphically, the three possible (2-dimensional) cases are shown here. The idea is to find where the lines intersect.

1. One, Unique Solution 2. Infinite Solutions 3. No Solution Case 1: One, Unique Solution
In this case, the two lines intersect at exactly one point. This is the easiest, most common and the best case possible. The other two cases are usually considered special situations.

Case 2: Infinite Solutions
In this case, the two lines are exactly the same. In the plot above, it looks like we have only one line. However, when we have two equations and we plot them, the lines are the same and so in graphs it looks like there is only one line. Since the lines are the same, the lines intersect at every point and, therefore, we say there are an infinite number of solutions.

Case 3: No Solution
When the lines are parallel, they will never intersect. So we say there is no solution since there is no point where they intersect.

Note: Some books and instructors switch the two cases above and call case 2 no solution and case 3 infinite solutions. The order does not matter, so it is best to follow your instructor and book.

Here is another good video showing all three cases, side-by-side including graphs.

### Khan Academy - Solving systems of equations (2x2), the special solution cases [9min-11secs]

Overview of Techniques

Solving systems of equations can be intuitively thought of as finding where the equations intersect. This is a great way to get a feel for what is happening. There are 4 main ways to solve systems of equations.
1. graphing
2. substitution
3. elimination
4. using matrices: Gaussian Elimination/Row Reduction, inverse matrices and Cramer's Rule.

This introduction video goes into a little more detail on these 4 techniques. The presenter actually crosses out the use of matrices option but on this site we cover that technique on other pages.

### Brightstorm - Introduction to Systems of Equations [3min-14secs]

video by Brightstorm

To check your answer, just plug the point into all of the original equations. Using the original equations is extremely important, since there could be a mistake at any point. And make sure and check all of the equations, since your answer may work in one but not all of them.

Okay, let's start with solving these problems by discussing 3 basic techniques, graphing, substitution and elimination.

Solve by Graphing

If you have only two equations and two unknowns, graphing can sometimes be used to solve the system of equations. We say 'sometimes' since this technique is not very exact and works only if you can graph very, very accurately and read the graph accurately. But for starters it is good to learn to do this to get a feel for what you are actually doing with the equations.
Here is a good video to get you started. He explains how to solve by graphing while doing an example. He also shows how to check your answer, an important step when working these kinds of problems.

### Khan Academy - Graphing Systems of Equations [6min-34secs]

Okay, let's work some practice problems before we go on. Solve these linear systems by graphing.

$$5x-y=6$$
$$2x+y=8$$

Problem Statement

$$5x-y=6$$
$$2x+y=8$$

Solution

### 1705 video

video by PatrickJMT

$$y=-2x+2$$
$$y=-6x-2$$

Problem Statement

$$y=-2x+2$$
$$y=-6x-2$$

Solution

### 1706 video

video by MIP4U

$$y=-x+3$$
$$2x-2y=10$$

Problem Statement

$$y=-x+3$$
$$2x-2y=10$$

Solution

### 1708 video

video by MIP4U

$$2x+y=4$$
$$y=-2x-5$$

Problem Statement

$$2x+y=4$$
$$y=-2x-5$$

Solution

### 1709 video

video by MIP4U

$$y=x+3$$
$$y=-x+3$$

Problem Statement

$$y=x+3$$
$$y=-x+3$$

Solution

### 2128 video

$$y=3x-6$$
$$y=-x+6$$

Problem Statement

$$y=3x-6$$
$$y=-x+6$$

Solution

Solve by Substitution

A second technique to solve systems of equations is substitution. Just like graphing, this technique is limited to two equations with two unknowns since higher order systems become quite complicated very quickly. Also this technique will often produce fractions very quickly, making the algebra more difficult. However, this is a handy technique to learn since you will use it later on with more effect.
Here is a good video showing this technique using an example. He doesn't check his answer in this video, which is not something you want to skip. Also, notice that his answer contains fractions, so graphing would not be the way to solve this problem.

### MathOnPoint - Solving Linear Systems Substitution Method [7min-58secs]

video by MathOnPoint

Let's work some practice problems. Solve these linear systems using substitution.

$$2x+4y=4$$
$$y=x-2$$

Problem Statement

$$2x+4y=4$$
$$y=x-2$$

Solution

### 1710 video

video by PatrickJMT

$$x+3y=6$$
$$2x+6y=-12$$

Problem Statement

$$x+3y=6$$
$$2x+6y=-12$$

Solution

### 1711 video

video by PatrickJMT

$$2x-3y=6$$
$$4x-6y=12$$

Problem Statement

$$2x-3y=6$$
$$4x-6y=12$$

Solution

### 1712 video

video by PatrickJMT

$$x+2y=9$$
$$3x+5y=20$$

Problem Statement

$$x+2y=9$$
$$3x+5y=20$$

Solution

### 1713 video

$$y=3x$$
$$2x+y=-10$$

Problem Statement

$$y=3x$$
$$2x+y=-10$$

Solution

### 1715 video

video by MIP4U

$$x-2y=1$$
$$3x-6y=-18$$

Problem Statement

$$x-2y=1$$
$$3x-6y=-18$$

Solution

### 1716 video

video by MIP4U

$$x-2y=7$$
$$2y-3x=-1$$

Problem Statement

$$x-2y=7$$
$$2y-3x=-1$$

Solution

### 1717 video

video by MIP4U

$$2x-y=-5$$
$$4x-2y=-10$$

Problem Statement

$$2x-y=-5$$
$$4x-2y=-10$$

Solution

### 1718 video

video by MIP4U

$$x+3y=12; 2x+y=6$$

Problem Statement

$$x+3y=12; 2x+y=6$$

Solution

### 2129 video

video by MathOnPoint

Here is a little more difficult problem that consists of 3 equations and 3 unknowns. Try substitution here as well.

$$7x-4y-2z=7; 8x-2y-8z=1; x+2y-6z=-6$$

Problem Statement

$$7x-4y-2z=7; 8x-2y-8z=1; x+2y-6z=-6$$

Solution

### 1714 video

video by PatrickJMT

Solve by Elimination

This is the best technique to solve systems of equations since it works all the time, you can control the numbers to avoid fractions until near the end of the solution and it will prepare you for the fourth technique using matrices. So it is important to learn this technique well.
Here is a good video explaining this technique while working an example, AND he explains why this technique works.

### Khan Academy - Solving Systems of Equations by Elimination [12min-43secs]

Okay, how about some practice problems. Solve these linear systems using elimination.

$$2x+3y=4$$
$$-2x+7y=16$$

Problem Statement

$$2x+3y=4$$
$$-2x+7y=16$$

Solution

### 1719 video

video by PatrickJMT

$$x-3y=6$$
$$4x-3y=10$$

Problem Statement

$$x-3y=6$$
$$4x-3y=10$$

Solution

### 1720 video

video by PatrickJMT

$$4x-2y=16$$
$$5x+2y=11$$

Problem Statement

$$4x-2y=16$$
$$5x+2y=11$$

Solution

### 1721 video

video by MIP4U

$$4x+3y=8$$
$$x-3y=7$$

Problem Statement

$$4x+3y=8$$
$$x-3y=7$$

Solution

### 1724 video

video by MIP4U

$$3x+5y=4$$
$$-2x+3y=10$$

Problem Statement

$$3x+5y=4$$
$$-2x+3y=10$$

Solution

### 1725 video

video by MIP4U

$$2x-3y=-1$$
$$-4x+6y=5$$

Problem Statement

$$2x-3y=-1$$
$$-4x+6y=5$$

Solution

### 1726 video

video by MIP4U

$$5x+2y=4$$
$$5x+2y=-2$$

Problem Statement

$$5x+2y=4$$
$$5x+2y=-2$$

Solution

### 1727 video

video by MIP4U

$$3x+y=-10; 7x+5y=-18$$

Problem Statement

$$3x+y=-10; 7x+5y=-18$$

Solution

### 1728 video

video by MIP4U

$$2x=-6y+8$$
$$3x-5y=2$$

Problem Statement

$$2x=-6y+8$$
$$3x-5y=2$$

$$(13/7,5/7)$$

Problem Statement

$$2x=-6y+8$$
$$3x-5y=2$$

Solution

The instructor in this video runs out of time and does not finish the problem. He gets $$y=5/7$$ and then stops. Using his work, here is how to get the final answer.
Substituting $$y=5/7$$ into the first original equation, we have
$$\begin{array}{rcl} 2x & = & -6y+8 \\ 2x & = & -6(5/7)+8 \\ 2x & = & -30/7+56/7 \\ 2x & = & 26/7 \\ x & = & 13/7 \end{array}$$

$$(13/7,5/7)$$

Here are a few 3x3 systems but elimination will work with these systems as well.

$$4x-4y+8z=20; 8x+4y-4z=4; 12x-8y-12z=-40$$

Problem Statement

$$4x-4y+8z=20; 8x+4y-4z=4; 12x-8y-12z=-40$$

Solution

### 1722 video

video by PatrickJMT

$$2x-y+z=3; 5x+2y-3z=1; 2x+y-z=2$$

Problem Statement

$$2x-y+z=3; 5x+2y-3z=1; 2x+y-z=2$$

Solution

### 1723 video

video by PatrickJMT

Next

If you haven't already, work some practice problems from the sections above. After that, your next step is to learn how to use matrices to solve systems of equations. Gaussian Elimination is the best place to start. You will learn some general information about matrices, what they are and how to work with them. Then we show you how to solve systems of equations using basic matrix operations. Much of what you learned on this page will help you, so make sure you are comfortable with these techniques, especially elimination, before moving on.

### solving linear systems 17calculus youtube playlist

Really UNDERSTAND Precalculus

### Calculus Topics Listed Alphabetically

Single Variable Calculus

Multi-Variable Calculus

### Search Practice Problems

Do you have a practice problem number but do not know on which page it is found? If so, enter the number below and click 'page' to go to the page on which it is found or click 'practice' to be taken to the practice problem. The 17Calculus and 17Precalculus iOS and Android apps are no longer available for download. If you are still using a previously downloaded app, your app will be available until the end of 2020, after which the information may no longer be available. However, do not despair. All the information (and more) is now available on 17calculus.com for free.

How to Develop a Brilliant Memory Week by Week: 50 Proven Ways to Enhance Your Memory Skills Save 20% on Under Armour Plus Free Shipping Over \$49! Shop Amazon - Sell Us Your Books - Get up to 80% Back When using the material on this site, check with your instructor to see what they require. Their requirements come first, so make sure your notation and work follow their specifications.

DISCLAIMER - 17Calculus owners and contributors are not responsible for how the material, videos, practice problems, exams, links or anything on this site are used or how they affect the grades or projects of any individual or organization. We have worked, to the best of our ability, to ensure accurate and correct information on each page and solutions to practice problems and exams. However, we do not guarantee 100% accuracy. It is each individual's responsibility to verify correctness and to determine what different instructors and organizations expect. How each person chooses to use the material on this site is up to that person as well as the responsibility for how it impacts grades, projects and understanding of calculus, math or any other subject. In short, use this site wisely by questioning and verifying everything. If you see something that is incorrect, contact us right away so that we can correct it.