Finding the Matrix Inverse

Oct 13, 2025 by wbrum.org 127 views

Introduction

In this article, we will explore how to solve a system of linear equations using a matrix inverse. This method is particularly useful when dealing with systems of equations that have a large number of variables. We will use the given system of equations as an example to demonstrate the process.

The System of Equations

The given system of equations is:

{ \begin{array}{l} z + y = 10 \\ 2x - z = 9 \\ y - 3z = -5 \end{array} \}

We can represent this system of equations in matrix form as:

{ \begin{bmatrix} 1 & 1 & 0 \\ 2 & 0 & -1 \\ 0 & 1 & -3 \end{bmatrix} \begin{bmatrix} x \\ y \\ z \end{bmatrix} = \begin{bmatrix} 10 \\ 9 \\ -5 \end{bmatrix} \}

The Coefficient Matrix

The coefficient matrix $A$ is the matrix that contains the coefficients of the variables in the system of equations. In this case, the coefficient matrix is:

{ A = \begin{bmatrix} 1 & 1 & 0 \\ 2 & 0 & -1 \\ 0 & 1 & -3 \end{bmatrix} \}

Finding the Matrix Inverse

To solve the system of equations using a matrix inverse, we need to find the inverse of the coefficient matrix $A$ . The inverse of a matrix $A$ is denoted as $A^{-1}$ and is defined as:

{ A^{-1} = \frac{1}{\det(A)} \text{adj}(A) \}

where $\det(A)$ is the determinant of the matrix $A$ and $\text{adj}(A)$ is the adjugate matrix of $A$ .

Calculating the Determinant

To find the determinant of the matrix $A$ , we can use the formula:

{ \det(A) = a(ei - fh) - b(di - fg) + c(dh - eg) \}

where $a$ , $b$ , $c$ , $d$ , $e$ , $f$ , $g$ , $h$ , and $i$ are the elements of the matrix $A$ .

Applying this formula to the matrix $A$ , we get:

{ \det(A) = 1(0 - (-3)) - 1(2(-3) - 0) + 0(2(1) - 0) \}

{ \det(A) = 1(3) - 1(-6) + 0(2) \}

{ \det(A) = 3 + 6 \}

{ \det(A) = 9 \}

Calculating the Adjugate Matrix

The adjugate matrix of $A$ is denoted as $\text{adj}(A)$ and is defined as:

{ \text{adj}(A) = \begin{bmatrix} ei - fh & ch - bi & bf - ce \\ fg - di & ai - cg & cd - af \\ dh - eg & bg - ah & ae - bd \end{bmatrix} \}

where $a$ , $b$ , $c$ , $d$ , $e$ , $f$ , $g$ , $h$ , and $i$ are the elements of the matrix $A$ .

Applying this formula to the matrix $A$ , we get:

{ \text{adj}(A) = \begin{bmatrix} 0 - (-3) & 0 - 0 & 0 - 0 \\ 2 - 0 & 1 - 0 & 0 - 2 \\ 0 - 0 & 1 - 0 & 1 - 2 \end{bmatrix} \}

{ \text{adj}(A) = \begin{bmatrix} 3 & 0 & 0 \\ 2 & 1 & -2 \\ 0 & 1 & -1 \end{bmatrix} \}

Finding the Matrix Inverse

Now that we have the determinant and the adjugate matrix, we can find the matrix inverse $A^{-1}$ using the formula:

{ A^{-1} = \frac{1}{\det(A)} \text{adj}(A) \}

Substituting the values of the determinant and the adjugate matrix, we get:

{ A^{-1} = \frac{1}{9} \begin{bmatrix} 3 & 0 & 0 \\ 2 & 1 & -2 \\ 0 & 1 & -1 \end{bmatrix} \}

{ A^{-1} = \begin{bmatrix} \frac{1}{3} & 0 & 0 \\ \frac{2}{9} & \frac{1}{9} & -\frac{2}{9} \\ 0 & \frac{1}{9} & -\frac{1}{9} \end{bmatrix} \}

Solving the System of Equations

Now that we have the matrix inverse $A^{-1}$ , we can solve the system of equations using the formula:

{ \begin{bmatrix} x \\ y \\ z \end{bmatrix} = A^{-1} \begin{bmatrix} 10 \\ 9 \\ -5 \end{bmatrix} \}

Substituting the values of the matrix inverse and the constant matrix, we get:

{ \begin{bmatrix} x \\ y \\ z \end{bmatrix} = \begin{bmatrix} \frac{1}{3} & 0 & 0 \\ \frac{2}{9} & \frac{1}{9} & -\frac{2}{9} \\ 0 & \frac{1}{9} & -\frac{1}{9} \end{bmatrix} \begin{bmatrix} 10 \\ 9 \\ -5 \end{bmatrix} \}

Multiplying the matrices, we get:

{ \begin{bmatrix} x \\ y \\ z \end{bmatrix} = \begin{bmatrix} \frac{10}{3} \\ \frac{2}{9} + \frac{9}{9} - \frac{10}{9} \\ \frac{9}{9} - \frac{5}{9} \end{bmatrix} \}

{ \begin{bmatrix} x \\ y \\ z \end{bmatrix} = \begin{bmatrix} \frac{10}{3} \\ \frac{1}{9} \\ \frac{4}{9} \end{bmatrix} \}

Therefore, the solution to the system of equations is:

{ x = \frac{10}{3} \}

{ y = \frac{1}{9} \}

{ z = \frac{4}{9} \}

Conclusion

Q: What is a matrix inverse?

A: A matrix inverse is a matrix that, when multiplied by the original matrix, results in the identity matrix. In other words, if we have a matrix A, its inverse is denoted as A^-1, and it satisfies the equation AA^-1 = A^-1A = I, where I is the identity matrix.

Q: Why do we need to find the matrix inverse to solve a system of equations?

A: We need to find the matrix inverse to solve a system of equations because it allows us to easily find the solution to the system. By multiplying the matrix inverse by the constant matrix, we can obtain the solution to the system.

Q: How do we find the matrix inverse of a matrix?

A: To find the matrix inverse of a matrix, we need to follow these steps:

Find the determinant of the matrix.
Find the adjugate matrix of the matrix.
Divide the adjugate matrix by the determinant to obtain the matrix inverse.

Q: What is the determinant of a matrix?

A: The determinant of a matrix is a scalar value that can be used to determine the invertibility of the matrix. If the determinant is non-zero, the matrix is invertible.

Q: How do we find the determinant of a matrix?

A: To find the determinant of a matrix, we can use the formula:

det(A) = a(ei - fh) - b(di - fg) + c(dh - eg)

where a, b, c, d, e, f, g, h, and i are the elements of the matrix A.

Q: What is the adjugate matrix of a matrix?

A: The adjugate matrix of a matrix is a matrix that contains the cofactors of the original matrix. The cofactors are obtained by replacing each element of the original matrix with its minor, and then multiplying the minor by either 1 or -1, depending on its position.

Q: How do we find the adjugate matrix of a matrix?

A: To find the adjugate matrix of a matrix, we can use the formula:

adj(A) = [[ei - fh, ch - bi, bf - ce], [fg - di, ai - cg, cd - af], [dh - eg, bg - ah, ae - bd]]

where a, b, c, d, e, f, g, h, and i are the elements of the matrix A.

Q: What is the relationship between the matrix inverse and the adjugate matrix?

A: The matrix inverse is related to the adjugate matrix by the formula:

A^-1 = (1/det(A)) * adj(A)

where det(A) is the determinant of the matrix A, and adj(A) is the adjugate matrix of A.

Q: Can we use the matrix inverse method to solve any system of equations?

A: No, we cannot use the matrix inverse method to solve any system of equations. The matrix inverse method is only applicable to systems of equations that have a unique solution and a non-singular coefficient matrix.

Q: What are some common applications of the matrix inverse method?

A: The matrix inverse method has many applications in various fields, including:

Linear algebra
Calculus
Physics
Engineering
Computer science

Some common applications of the matrix inverse method include:

Solving systems of linear equations
Finding the inverse of a matrix
Calculating the determinant of a matrix
Solving linear systems with multiple variables

Q: How do we choose the correct method to solve a system of equations?

A: To choose the correct method to solve a system of equations, we need to consider the following factors:

The number of variables in the system
The type of equations in the system (linear, quadratic, etc.)
The complexity of the system
The desired solution (unique, multiple, etc.)

Based on these factors, we can choose the most suitable method to solve the system of equations.

Q: What are some common mistakes to avoid when using the matrix inverse method?

A: Some common mistakes to avoid when using the matrix inverse method include:

Not checking the invertibility of the coefficient matrix
Not calculating the determinant correctly
Not finding the adjugate matrix correctly
Not multiplying the matrix inverse by the constant matrix correctly

By avoiding these common mistakes, we can ensure that we obtain the correct solution to the system of equations using the matrix inverse method.