The Method of Substitution

Oct 13, 2025 by wbrum.org 109 views

Introduction

Systems of linear equations are a fundamental concept in mathematics, and solving them is a crucial skill for students and professionals alike. In this article, we will focus on solving a system of two linear equations with two variables. We will use the method of substitution and elimination to find the solution.

What are Systems of Linear Equations?

A system of linear equations is a set of two or more linear equations that involve two or more variables. Each equation is in the form of ax + by = c, where a, b, and c are constants, and x and y are variables. The goal of solving a system of linear equations is to find the values of the variables that satisfy all the equations in the system.

The Method of Substitution

One of the methods used to solve systems of linear equations is the method of substitution. This method involves solving one equation for one variable and then substituting that expression into the other equation. Let's use this method to solve the system of equations:

{ \begin{cases} 4x + 5y = 3 \\ 3x - 2y = 8 \end{cases} \}

Step 1: Solve the First Equation for x

We can solve the first equation for x by subtracting 5y from both sides:

4x = 3 - 5y

Next, we can divide both sides by 4 to solve for x:

x = (3 - 5y) / 4

Step 2: Substitute the Expression for x into the Second Equation

Now that we have an expression for x, we can substitute it into the second equation:

3((3 - 5y) / 4) - 2y = 8

To simplify this equation, we can multiply both sides by 4 to eliminate the fraction:

3(3 - 5y) - 8y = 32

Expanding the left-hand side of the equation, we get:

9 - 15y - 8y = 32

Combine like terms:

-23y = 23

Step 3: Solve for y

Now that we have a simple equation, we can solve for y by dividing both sides by -23:

y = -1

Step 4: Find the Value of x

Now that we have the value of y, we can substitute it into the expression for x that we found in Step 1:

x = (3 - 5(-1)) / 4

Simplifying the expression, we get:

x = (3 + 5) / 4

x = 8 / 4

x = 2

The Solution

Therefore, the solution to the system of equations is x = 2 and y = -1.

The Method of Elimination

Another method used to solve systems of linear equations is the method of elimination. This method involves adding or subtracting the equations in the system to eliminate one of the variables. Let's use this method to solve the system of equations:

{ \begin{cases} 4x + 5y = 3 \\ 3x - 2y = 8 \end{cases} \}

Step 1: Multiply the Equations by Necessary Multiples

To eliminate one of the variables, we need to multiply the equations by necessary multiples. Let's multiply the first equation by 2 and the second equation by 5:

8x + 10y = 6

15x - 10y = 40

Step 2: Add the Equations

Now that we have the equations multiplied by necessary multiples, we can add them to eliminate one of the variables:

(8x + 10y) + (15x - 10y) = 6 + 40

Combine like terms:

23x = 46

Step 3: Solve for x

Now that we have a simple equation, we can solve for x by dividing both sides by 23:

x = 46 / 23

x = 2

Step 4: Find the Value of y

Now that we have the value of x, we can substitute it into one of the original equations to find the value of y. Let's use the first equation:

4x + 5y = 3

Substitute x = 2:

4(2) + 5y = 3

Simplify the equation:

8 + 5y = 3

Subtract 8 from both sides:

5y = -5

Divide both sides by 5:

y = -1

The Solution

Therefore, the solution to the system of equations is x = 2 and y = -1.

Conclusion

Q: What is a system of linear equations?

A: A system of linear equations is a set of two or more linear equations that involve two or more variables. Each equation is in the form of ax + by = c, where a, b, and c are constants, and x and y are variables.

Q: What are the methods used to solve systems of linear equations?

A: There are two main methods used to solve systems of linear equations: the method of substitution and the method of elimination.

Q: What is the method of substitution?

A: The method of substitution involves solving one equation for one variable and then substituting that expression into the other equation.

Q: What is the method of elimination?

A: The method of elimination involves adding or subtracting the equations in the system to eliminate one of the variables.

Q: How do I choose which method to use?

A: You can choose which method to use based on the coefficients of the variables in the equations. If the coefficients of one variable are the same in both equations, you can use the method of elimination. If the coefficients of one variable are different in both equations, you can use the method of substitution.

Q: What if I have a system of linear equations with three or more variables?

A: If you have a system of linear equations with three or more variables, you can use the method of substitution or elimination to solve for two variables, and then use the values of those variables to solve for the third variable.

Q: What if I have a system of linear equations with no solution?

A: If you have a system of linear equations with no solution, it means that the equations are inconsistent. This can happen if the equations are contradictory, such as 2x + 3y = 5 and 2x + 3y = 10.

Q: What if I have a system of linear equations with infinitely many solutions?

A: If you have a system of linear equations with infinitely many solutions, it means that the equations are dependent. This can happen if the equations are equivalent, such as 2x + 3y = 5 and 4x + 6y = 10.

Q: How do I graph a system of linear equations?

A: To graph a system of linear equations, you can plot the equations on a coordinate plane and find the point of intersection. The point of intersection represents the solution to the system of equations.

Q: What are some real-world applications of solving systems of linear equations?

A: Solving systems of linear equations has many real-world applications, such as:

Finding the intersection of two lines in a coordinate plane
Determining the cost of producing a product based on the number of units produced and the cost per unit
Finding the maximum or minimum value of a function
Solving problems in physics, engineering, and economics

Conclusion

Solving systems of linear equations is a fundamental concept in mathematics, and it has many real-world applications. In this article, we have answered some frequently asked questions about solving systems of linear equations, including the methods used to solve them, how to choose which method to use, and some real-world applications. We hope that this article has provided a clear and concise explanation of how to solve systems of linear equations.