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Algebra II: High School23 chapters | 203 lessons

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Lesson Transcript

Instructor:
*Yuanxin (Amy) Yang Alcocer*

Amy has a master's degree in secondary education and has taught math at a public charter high school.

After watching this video lesson, you will be able to solve a linear system in three variables using the elimination method. Learn the simple operations that are needed to solve your system.

In this video lesson, we talk about **linear systems in three variables**. These are collections of three linear equations in three variables. Each of our equations will have three variables, but none of the variables will have exponents with them.

The most common variables that we will see are the letters *x*, *y*, and *z*. Even though we have three variables, each equation may not have all three listed. If we don't see a variable listed, we note that variable as having a coefficient of zero.

When you graph each equation, you will end up with a flat plane. When the three planes intersect at one point, then we say that the system has a solution that we can solve for. You will come across these systems as you progress in your math. Let's try and solve one of them using what we call the elimination method:

We see that we have a linear system of three equations in three variables. We see that the first equation has three variables in it, but the second and third equations don't. In the second equation, the coefficient linked to *z* is a 0; in the third equation, the coefficient linked with the *y* is a 0.

The **elimination method** involves combining two of our equations together to try and eliminate at least one of our variables. To combine two equations, we may have to multiply one equation by a particular number so that when we add the two equations together, at least one of the variables disappears. Let's see how this works with our system.

We look at our equations, and we see that the first and second equation both have an *x*. I can multiply the second equation by a -1 and then add that to the first equation so that the *x*s disappear. Multiplying the second equation by a -1 gives me -*x* - 2*y* = -5. Adding this to the first equation, gives me -*y* + *z* = 1. Okay, so far so good. I have a new equation without the *x* in it. I now have a total of four equations I can work with.

Let's keep going. This time I can multiply my first equation by a -2 and then add it to the third equation to get rid of the *x* again. Multiplying the first equation by -2 gives me -2*x* - 2*y* - 2*z* = -12. Adding this to the third equation gives me -2*y* - *z* = -7. This is another new equation. I now have five total equations I can use.

Now I see that my fourth and fifth equations have both a *y* and a *z*. I look at them and I see that I can add them right away to get rid of the *z*. So, adding them I get -3*y* = -6. Hey, look at that! I can easily solve this new equation!

To finish the problem, we will first solve our last equation, the -3*y* = -6. Then, we will use this answer to help us find the rest. We solve -3*y* = -6 by dividing by -3 to get *y* = 2. Good!

I can plug this into either the fourth or fifth equation to find my *z*. The fourth equation looks easier, so I choose this one to plug my *y* into. I get -2 + *z* = 1. I solve it by adding 2 to both sides. I get *z* = 3. Awesome!

I can now finish the problem by plugging in both *y* and *z* into the very first equation: *x* + 2 + 3 = 6. Subtracting 2 and 3 from both sides, I get *x* = 1. I have solved my linear system in three variables! My answer is *x* = 1, *y* = 2, and *z* = 3. I can also write this as (1, 2, 3).

Let's review what we've learned now. **Linear systems in three variables** are collections of three linear equations in three variables. In such a system, all the equations will have three variables. If a variable is not seen, then that variable has a coefficient of zero. The most common variables that you will see are *x*, *y*, and *z*.

To solve such a system, we can use the elimination method. The **elimination method** involves combining pairs of equations so that we can eliminate at least one of the variables. To combine equations, we may have to multiply one equation by a number so that when it is added to the second, at least one of the variables disappears.

We keep repeating this process until we get to the point where we have an equation with only one variable. Then, we solve that equation for that variable. We use this answer and substitute it into another equation that we can solve after subbing in the answer. Then, we use our two answers to find the third and final answer. Our final answer will have three numbers: one for *x*, one for *y*, and one for *z*.

You should have the ability to do the following after this lesson:

- Describe a linear system in three variables
- Explain how to solve this system using the elimination method

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Algebra II: High School23 chapters | 203 lessons

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