Main content

## Algebra (all content)

### Course: Algebra (all content) > Unit 5

Lesson 3: Equivalent systems of equations and the elimination method- Systems of equations with elimination: King's cupcakes
- Systems of equations with elimination: x-4y=-18 & -x+3y=11
- Systems of equations with elimination
- Systems of equations with elimination: potato chips
- Systems of equations with elimination (and manipulation)
- Systems of equations with elimination challenge
- Why can we subtract one equation from the other in a system of equations?
- Worked example: equivalent systems of equations
- Worked example: non-equivalent systems of equations
- Reasoning with systems of equations
- Solving systems of equations by elimination (old)
- Elimination method review (systems of linear equations)
- Equivalent systems of equations review

© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Equivalent systems of equations review

Two systems of equations are equivalent if they have the same solution(s). This article reviews how to tell if two systems are equivalent.

Systems of equations that have the same solution are called

*equivalent systems*.Given a system of two equations, we can produce an equivalent system by replacing one equation by the sum of the two equations, or by replacing an equation by a multiple of itself.

In contrast, we can be sure that two systems of equations are

*not*equivalent if we know that a solution of the one is*not*a solution of the other.*Note: This idea of equivalent systems of equations pops up again in linear algebra. However, the examples and explanations in this article are geared to a high school algebra 1 class.*

## Example 1

We're given two systems of equations and asked if they're equivalent.

System A | System B |
---|---|

$\begin{aligned}-12x+9y=7\\\\9x-12y=6\end{aligned}$ | $\begin{aligned}-12x+9y=7\\\\3x-4y=2\end{aligned}$ |

If we multiply the second equation in System B by 3, we get:

Replacing the second equation of System B with this new equation, we get an equivalent system:

Whoa! Look at that! This system is the same as System A, which means system A is equivalent to System B.

*Want to learn more about equivalent systems of equations? Check out this video.*

## Example 2

We're given two systems of equations and asked if they're equivalent.

System A | System B |
---|---|

$\begin{aligned}-9x-4y&=5\\\\2x+5y&=-4\end{aligned}$ | $\begin{aligned}-7x+y&=1\\\\2x+5y&=-4\end{aligned}$ |

Interestingly, if we sum the equations in System A, we get:

Replacing the first equation in System A with this new equation, we get a system that's equivalent to System A:

Lo and behold! This is System B, which means that System A is equivalent to System B.

## Example 3

We're given two systems and asked to prove that they aren't equivalent by finding a solution of one that is not a solution of the other.

System A | System B |
---|---|

$\begin{aligned}-4x+10y&=1\\\\-1x-2y&=-3\end{aligned}$ | $\begin{aligned}-9x-y&=8\\\\-1x-2y&=4\end{aligned}$ |

Notice how the coefficients for x and y in the second equations of both systems are the same. However, the constant terms in the two equations are different!

Whichever pair of values for x and y that makes System A true will make System B false, and vice versa.

For example, x, equals, 1, y, equals, 1 is a solution to the second equation in System A, but it's not a solution to the second equation in System B.

System A and System B are not equivalent.

*Want to learn more about non-equivalent systems of equations? Check out this video*

## Want to join the conversation?

- When we subtract or add one equation to the second equation in a system of equations, the sum or difference produces an entirely new line, a line with a new slope and y-intercept. This is contrary to my instincts because we are adding or subtracting equivalent values when we add or subtract the second equation from the first. Could you explain to me how we get an entirely unique line as a result? How is it that adding and subtracting equivalent values produces something new?(10 votes)
- What if it had fractions?(7 votes)
- change fractions into whole numbers. remember, whenever you do something to one-side, you will have to do to another-side to maintain the balance (equivalent).

For example, is A,B equivalent to C,D?

----------------------------------------

A) -4x + 10y = 1

B) -5x - 11y = 7

----------------------------------------

C) -9x -y = 8

D) -5/10x +5/4y = 125/1000

----------------------------------------

like Khan always say, try to solve this yourself.

the answer is yes, these 2 systems are equivalent.

C is equivalent to A&B combined.

D times 8 is equivalent to A.

Hope it helps!

**I'm not a pro in Math, in fact, I've just started learning Math about 2 months ago. So, please elaborate if I said something wrong, thanks!**(11 votes)

- The process in which you determine which system of equations are equivalent is confusing. In the examples they tell you which systems are equivalent but it doesn't really explain how you can figure that out.(8 votes)
- Equivalent systems have either: the same lines (obviously); multiples of the original lines; or combinations of the original lines (adding or subtracting two original lines).(3 votes)

- "Replacing the first equation in System A with this new equation, we get a system that's equivalent to System A:"

This sentence from example 2 makes no sense.(4 votes)- I think he meant "equivalent to System B"(1 vote)

- what do you do if you click the wrong one? :((3 votes)
- It's not a test so I just clicked the right one and then clicked check again. If you do that it doesn't even show you ever got it wrong.(2 votes)

- ayye....can any one help me wth some steps to this...OH and my name is xxxevolution(0 votes)
- XXevolution,

This is the end of the unit. Unless you can ask about specific steps, at this point, I would suggest that you go back through the videos and exercises.

If you have a specific step on which you need help, please explain which step it is.(7 votes)

- This was not how I learned it in class. Why does it say that only the solution needs to be the same for two systems of equations to be equivalent? How I learned it, the two systems need to be the same in order to be equivalent. If I learned it wrong please tell me but otherwise help me understand.(2 votes)
- example: an equivalent system that consist of equation 2x+3y=5 could be any multiplication by any number of that equation, lets say for example if you want to multiply it by 3 then we get the second equation of the system to be 6x+9y=15 which is 3(2x+3y=5)(3 votes)

- Why, when you add/subtract one equation from the other on, does it make an equation that is considered to be equal?(1 vote)
- The properties of equality allow us to add/subtract the same value to both sides of an equation and still have an equivalent equation.

The 2 sides of an equation are equivalent. So, when you add/subtract one equation to another, you are adding/subtracting equal values (same values) to both sides of the equation. The values just don't look the same.

Hope this helps.(4 votes)

- In general, can you add the teacher’s two equations and solve for y and then do the same thing for the other two and see how they plot?(2 votes)
- How are these going to be useful in real life?(2 votes)
- Systems of equations are used in economics, for example supply and demand curves. It is used in travel and when planes are extimated to arrive. You can use it to compare insurance, phone bills, etc.(1 vote)