Substitution Method
One of the methods to solve a system of linear equations in two variables algebraically is the substitution method. In this method, we find the value of any one of the variables by isolating it on one side and taking every other term on the other side of the equation. Then we substitute that value in the second equation. It involves simple steps to find the values of variables of a system of linear equations by substitution method. Let's learn about it in detail in this article.
1.  What is Substitution Method? 
2.  Solving Systems of Equations by Substitution Method 
3.  Difference Between Elimination and Substitution Method 
4.  FAQs on Substitution Method 
What is Substitution Method?
The substitution method is a simple way to solve a system of linear equations algebraically and find the solutions of the variables. As the name suggests, it involves finding the value of xvariable in terms of yvariable from the first equation and then substituting or replacing the value of xvariable in the second equation. In this way, we can solve and find the value of the yvariable. And at last, we can put the value of y in any of the given equations to find x This process can be interchanged as well where we first solve for x and then solve for y.
Substitution Method Definition
The substitution method is one of the algebraic methods to solve simultaneous linear equations. It involves substituting the value of any one of the variables from one equation into the other equation. The other two algebraic methods of solving linear equations are the elimination method and the cross multiplication method. Apart from the algebraic method, we can also solve a system of linear equations graphically.
Let us take an example of solving two equations x2y=8 and x+y=5 using the substitution method.
Solving Systems of Equations by Substitution Method
The steps to apply or use the substitution method to solve a system of equations are given below:
 Step 1: Simplify the given equation by expanding the parenthesis if needed.
 Step 2: Solve any one of the equations for any one of the variables. You can use any variable based on the ease of calculation.
 Step 3: Substitute the obtained value of x or y in the other equation.
 Step 4: Now, simplify the new equation obtained using arithmetic operations and solve the equation for one variable.
 Step 5: Now, substitute the value of the variable from Step 4 in any of the given equations to solve for the other variable.
Here is an example of solving system of equations by using substitution method: 2x+3(y+5)=0 and x+4y+2=0.
Solution:
Step 1: So, here we can simplify the first equation to get 2x + 3y + 15 = 0. Now we have two equations as,
2x + 3y + 15 = 0 _____ (1)
x + 4y + 2 = 0 ______ (2)
Step 2: We are solving equation (2) for x. So, we get x = 4y  2.
Step 3: Substitute the obtained value of x in the equation (1). i.e., we are substituting x = 4y2 in the equation 2x + 3y + 15 = 0, we get, 2(4y2) + 3y + 15 = 0.
Step 4: Now, simplify the new equation. We get, 8y4+3y+15=0
5y + 11 = 0
5y = 11
y = 11/5
Step 5: Now, substitute the value of y in any of the given equations. Let us substitute the value of y in equation (2).
x + 4y + 2 = 0
x + 4 × (11/5) + 2 = 0
x + 44/5 + 2 = 0
x + 54/5 = 0
x = 54/5
Therefore, after solving the given system of equations by substitution method, we get x = 54/5 and y= 11/5.
Difference Between Elimination and Substitution Method
Both elimination and substitution methods are the ways to solve linear equations algebraically. When the substitution method becomes a little difficult to apply in equations involving large numbers or fractions, we can use the elimination method to ease out our calculations. Let us understand the difference between these two methods through the table given below:
Substitution Method  Elimination Method 

In this, we find the value of any one of the variables and substitute its value in the other equation.  In this method, we multiply or divide either one or both the equations by a number to make the coefficient of either xvariable or yvariable the same in both the equations. Then, we add or subtract the equations to eliminate the variable whose coefficient is the same. In this way, we find the value of one variable which can be substituted in any one of the equations to find the other variable too. 
It is better to use the substitution method when equations are either given in form or can be gotten into the form of x = ay + b and y = mx + n.  It is better to use the elimination method when the coefficient of any one of the terms is the same. For example, Ax+By+C=0 and Px+By+R=0. 
Important Notes on Substitution Method:
 To start with the substitution method, first, select the equation that has coefficient 1 for at least one of the variables and solve for the same variable (with coefficient 1). This makes the process easier.
 Before starting with the substitution method, combine all like terms (if any).
 After solving for one variable, we can select any of the given equations or any equation in the whole process to find the other variable.
 If we get any true statement like 3 = 3, 0 = 0, etc while solving using the substitution method, then it means that the system has infinitely many solutions.
 If we get any false statement like 3 = 2, 0 = 1, etc while solving using the substitution method, then it means that the system has no solution.
ā Related Topics:
Check these articles related to the substitution method.
Substitution Method Examples

Example 1: Sean was given two equations 5m−2n=17 and 3m+n=8. Can you how help him in finding the solution of these equations using the substitution method?
Solution: The given two equations are:
5m−2n=17 ____ (1)
3m+n=8 _____ (2)
The solution of the given two equations can be found by the following steps:
 From equation 2 we can find the value of n in terms of m, where n = 8  3m
 Substitute the value of n in equation 1. We get, 5m  2(83m)=17
5m  2(83m)=17
5m  16 + 6m =17
11m = 17 + 16
11m=33
m = 3
 Substitute the value of m in equation 2, we get, 3×3+n=8
9+n=8
n=89
n=1
Answer: Therefore, by substitution method, we have found out that m=3 and n=1.

Example 2: Jacky has two numbers such that the sum of two numbers is 20 and the difference between them is 10. Find the numbers by using the substitution method of solving linear equations.
Solution: Let the two numbers be x and y such that x>y. It is given that, x+y=20 ___ (1) and x−y=10 ___ (2). From equation 1, we get x = 20y. Substitute this value in equation 2 to find the value of y.
x−y=10
20yy=10
202y=10
2010=2y
10=2y
y=10/2 = 5
Now, substitute the value of y in equation 1, we get, x+5=20, which gives us x=15.
Answer: Therefore, the two numbers are 15 and 5.

Example 3: Solve the given system of linear equations by substitution method:
 2x  5 + 3x + y = 0 ___ (1)
3x + y = 11 ___ (2)
Solution: As we can see that the first equation can be further simplified by combining like terms. After simplifying it, we get x+y5=0. From this equation, let us find the value of x in terms of y, which is x = 5y. Now substitute this value in equation 2, we get 3(5y)+y=11.
153y+y=11
152y=11
1511=2y
4=2y
y=4/2=2
Now, let us substitute the value of y in equation 1. We get, x+25=0, which can be simplified to x = 3.
Answer: Therefore, by substitution method, we have x=3 and y=2.
FAQs on Substitution Method
What is the Substitution Method in Algebra?
In algebra, the substitution method is one of the ways to solve linear equations in two variables. In this method, we substitute the value of a variable found by one equation in the second equation. It is very easy to use when we have smaller numbers, but in the case of large numbers or fractional coefficients, it becomes tedious to apply the substitution method.
When would you Use the Substitution Method?
The substitution method can be applied to any pair of linear equations with two variables. It is advisable to use the substitution method when we have smaller coefficients in terms or when the equations are given in form x = ay+c and y=bx+p.
What do we Substitute in the Substitution Method?
In the substitution method, we substitute the value of one variable found by simplifying an equation in the other equation. For example, if there are two variables in the equations m and n, then we can first find the value of m in terms of n from any one of the equations, then we substitute that value in the second equation to get an answer of n. Then, we again substitute the value of n in any of the given equations.
What do the Substitution Method and the Elimination Method have in Common?
Both methods involve the process of substitution. In both methods, we find the value of one variable first and then substitute it in any of the given equations. So, this is common in both the elimination and the substitution method.
What is the First Step in the Substitution Method?
The first step in the substitution method is to find the value of any one of the variables from one equation in terms of the other variable. For example, if there are two equations x+y=7 and xy=8, then from the first equation we can find that x=7y. This is the first step of applying the substitution method.
What are the Steps for the Substitution Method?
The three simple steps for the substitution method are given below:
 Find the value of any one variable from any of the equations in terms of the other variable.
 Substitute it in the other equation and solve.
 Substitute the value of the second variable again in any of the equations.
How do you Use the Substitution Method with Two Variables?
With two variables let's say x and y, we first find the value of x in terms of y from any one of the equations given. Then, we substitute that value in the other equation to find the value of y. At last, we again substitute the value of y in any given equation to find x.
Is Substitution Method Only for Linear Equations?
No, substitution method can be applied for any type of equations. For example, the equations y = x^{2} and y = 3x + 4 can be solved by using the substitution method.
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