System of Linear Equations

In mathematics, the system of linear equations is the set of two or more linear equations involving the same variables. Here, linear equations can be defined as the equations of the first order, i.e., the highest power of the variable is 1. Linear equations can have one variable, two variables, or three variables. Thus, we can write linear equations with n number of variables. In this article, you will get the definition of the system of linear equations, different methods of solving these systems of linear equations and solved examples.

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System of Linear Equations Definition

Let

a₁₁x₁ + a₁₂x₂ + a₁₃x₃ + …. + a_1nx_n = b₁

a₂₁x₁ + a₂₂x₂ + a₂₃x₃ + …. + a_2nx_n = b₂

…………………………………..

…………………………………..

a_n1x₁ + a_n2x₂ + a_n3x₃ + …. + a_nnx_n = b_n

be a system of “n” linear equations in “n” variables x₁, x₂, x₃,…., x_n.

Where,

a₁₁, a₁₂, …, a₂₁, a₂₂,…, a_n1, a_n2,…, a_nn are the coefficients of variables, x₁, x₂,…., x_n.

System of Linear Equations Matrix

As we know, the system of linear equations can be written in matrix form. Thus, the system of linear equations in n variables can be written as:

(or)

AX = B

Here,

Thus, A is called the coefficient matrix.

Solutions to System of Linear Equations

Any set of values of x₁, x₂, x₂,…x_n which simultaneously satisfies the system of linear equations given above is called a solution of the system. If the system of equations has one or more solutions, the equations are called consistent. Also, if the system of equations does not admit any solution, then the equations are called inconsistent.

Consider the system of equations AX = B and these equations are said to be Homogeneous if B = 0 and Non-homogeneous if B ≠ 0.

How to Solve System of Linear Equations?

The following methods of solving system of linear equations AX = B, are applicable only when the coefficient matrix A is non-singular, i.e., |A| ≠ 0.

• Cramers method
• Inverse method
• Gauss-Jordan method
• Gauss Elimination method
• LU Decomposition method of factorisation (or) Method of Triangularisation

System of Linear Equations in Two Variables

The system of linear equations in two variables is the set of equations that contain only two variables. For example, 2x + 3y = 4; 3x + 5y = 12 are the system of equations in two variables. There are several methods of solving linear equations in two variables, such as:

1. Graphical method
2. Substitution Method
3. Elimination Method
4. Cross-Multiplication Method
5. Matrix method

Click here to learn more about linear equations in two variables.

Solving System of Linear Equations by Elimination

Let’s learn how to solve the system of linear equations by the elimination method here.

Consider the following system of linear equations in three variables.

2x – y + 3z = 9

x – 3y – 2z = 0

3x + 2y – z = -1

Step 1: Let us write the given equations in the form of AX = B.

Step 2: Now, write the augmented matrix [A : B].

Step 3: Using elementary row operations, eliminate the unknown x from all the equations, except the first. Eliminate y from the third equation. This can be done as follows.

Interchange R₁ and R₂, i.e., R₁ ↔ R₂.

Now, perform R₂ = R₂ – 2R₁ and R₃ = R₃ – 3R₁.

By performing R₃ = R₃ – (11/5)R₂, we get;

From the above matrix, we get;

x – 3y – 2z = 0….(1)

5y + 7z = 9….(2)

(-52/5)z = -104/5….(3)

Solving equation (3), we get z = 2.

Substituting z = 2 in equation (2), we get;

5y + 7(2) = 9

5y + 14 = 9

5y = 9 – 14

y = -5/5

y = -1

Substituting y = -1 and z = 2 in equation (1), we get;

x – 3(-1) – 2(2) = 0

x + 3 – 4 = 0

x = 1

Therefore, the solution set of given system of linear equations is x = 1, y = -1 and z = 2.

System of Linear Equations Problems

1. Solve the system of linear equations:
   x + y + z = 6
   3x – 2y – z = 4
   2x + 3y – 2z = 2
2. Solve the following system of linear equations by elimination method.
   3x + 5y + 6z = 7
   x + 3y – 2z = 5
   2x + 4y + 3z = 8
3. Solve: 3x – y + 14z = 7; 2x + 2y + 3z = 0; x – 12y – 18z = 33

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