Calculating the Determinant using Expansion by Minors

Oct 13, 2025 by wbrum.org 88 views

Introduction

In linear algebra, the determinant of a square matrix is a scalar value that can be used to describe the scaling effect of the matrix on a region of space. It is a fundamental concept in mathematics and has numerous applications in various fields, including physics, engineering, and computer science. In this article, we will discuss how to find the determinant of a 3x3 matrix, using the matrix $ A=\left[\begin{array}{lll}3 & 6 & 5 \ 2 & 1 & 8 \ 7 & 9 & 1\end{array}\right] $ as an example.

What is a Determinant?

The determinant of a square matrix is a scalar value that can be calculated using various methods, including the expansion by minors, the cofactor expansion, and the LU decomposition. The determinant of a matrix can be used to determine the invertibility of the matrix, the existence of solutions to a system of linear equations, and the stability of a linear system.

Method 1: Expansion by Minors

One of the most common methods for finding the determinant of a 3x3 matrix is the expansion by minors. This method involves expanding the determinant along a row or column of the matrix. To expand along a row, we multiply each element in the row by its minor, which is the determinant of the 2x2 matrix obtained by removing the row and column of the element. We then add or subtract the products of the elements and their minors, depending on their position in the row.

Calculating the Determinant using Expansion by Minors

To calculate the determinant of the matrix $ A=\left[\begin{array}{lll}3 & 6 & 5 \ 2 & 1 & 8 \ 7 & 9 & 1\end{array}\right] $ using the expansion by minors, we can expand along the first row. The minor of the element $ a_{11} $ is the determinant of the 2x2 matrix $ \left[\begin{array}{ll}1 & 8 \ 9 & 1\end{array}\right] $, which is $ (1)(1) - (8)(9) = -71 $. The minor of the element $ a_{12} $ is the determinant of the 2x2 matrix $ \left[\begin{array}{ll}2 & 8 \ 7 & 1\end{array}\right] $, which is $ (2)(1) - (8)(7) = -54 $. The minor of the element $ a_{13} $ is the determinant of the 2x2 matrix $ \left[\begin{array}{ll}2 & 1 \ 7 & 9\end{array}\right] $, which is $ (2)(9) - (1)(7) = 13 $. We can now calculate the determinant of the matrix $ A $ as follows:

\det(A) = a_{11}M_{11} - a_{12}M_{12} + a_{13}M_{13}

\det(A) = (3)(-71) - (6)(-54) + (5)(13)

\det(A) = -213 + 324 + 65

\det(A) = 176

Method 2: Cofactor Expansion

Another method for finding the determinant of a 3x3 matrix is the cofactor expansion. This method involves expanding the determinant along a row or column of the matrix, using the cofactors of the elements instead of the minors. The cofactor of an element is the determinant of the 2x2 matrix obtained by removing the row and column of the element, multiplied by $ (-1)^{i+j} $, where $ i $ and $ j $ are the row and column indices of the element.

Calculating the Determinant using Cofactor Expansion

To calculate the determinant of the matrix $ A=\left[\begin{array}{lll}3 & 6 & 5 \ 2 & 1 & 8 \ 7 & 9 & 1\end{array}\right] $ using the cofactor expansion, we can expand along the first row. The cofactor of the element $ a_{11} $ is the determinant of the 2x2 matrix $ \left[\begin{array}{ll}1 & 8 \ 9 & 1\end{array}\right] $, multiplied by $ (-1)^{1+1} = 1 $. The cofactor of the element $ a_{12} $ is the determinant of the 2x2 matrix $ \left[\begin{array}{ll}2 & 8 \ 7 & 1\end{array}\right] $, multiplied by $ (-1)^{1+2} = -1 $. The cofactor of the element $ a_{13} $ is the determinant of the 2x2 matrix $ \left[\begin{array}{ll}2 & 1 \ 7 & 9\end{array}\right] $, multiplied by $ (-1)^{1+3} = 1 $. We can now calculate the determinant of the matrix $ A $ as follows:

\det(A) = a_{11}C_{11} - a_{12}C_{12} + a_{13}C_{13}

\det(A) = (3)(-71) - (6)(-54) + (5)(13)

\det(A) = -213 + 324 + 65

\det(A) = 176

Method 3: LU Decomposition

Another method for finding the determinant of a 3x3 matrix is the LU decomposition. This method involves decomposing the matrix into the product of a lower triangular matrix and an upper triangular matrix. The determinant of the matrix can then be calculated as the product of the diagonal elements of the lower and upper triangular matrices.

Calculating the Determinant using LU Decomposition

To calculate the determinant of the matrix $ A=\left[\begin{array}{lll}3 & 6 & 5 \ 2 & 1 & 8 \ 7 & 9 & 1\end{array}\right] $ using the LU decomposition, we can decompose the matrix into the product of a lower triangular matrix $ L $ and an upper triangular matrix $ U $. The matrix $ L $ is given by:

L = \left[\begin{array}{lll}1 & 0 & 0 \\ l_{21} & 1 & 0 \\ l_{31} & l_{32} & 1\end{array}\right]

The matrix $ U $ is given by:

U = \left[\begin{array}{lll}u_{11} & u_{12} & u_{13} \\ 0 & u_{22} & u_{23} \\ 0 & 0 & u_{33}\end{array}\right]

We can now calculate the determinant of the matrix $ A $ as follows:

\det(A) = \det(L) \det(U)

\det(A) = (u_{11})(u_{22})(u_{33})

We can now calculate the determinant of the matrix $ A $ as follows:

\det(A) = (3)(1)(1)

\det(A) = 3

Conclusion

In this article, we have discussed three methods for finding the determinant of a 3x3 matrix: the expansion by minors, the cofactor expansion, and the LU decomposition. We have used the matrix $ A=\left[\begin{array}{lll}3 & 6 & 5 \ 2 & 1 & 8 \ 7 & 9 & 1\end{array}\right] $ as an example and calculated the determinant of the matrix using each method. The determinant of the matrix $ A $ is 176.

References

[1] Strang, G. (1988). Linear Algebra and Its Applications. 3rd ed. New York: Harper & Row.
[2] Lay, D. C. (2005). Linear Algebra and Its Applications. 3rd ed. Boston: Addison-Wesley.
[3] Anton, H. (1994). Elementary Linear Algebra. 7th ed. New York: Wiley.

Introduction

In our previous article, we discussed three methods for finding the determinant of a 3x3 matrix: the expansion by minors, the cofactor expansion, and the LU decomposition. In this article, we will answer some frequently asked questions about the determinant of a 3x3 matrix.

Q: What is the determinant of a matrix?

A: The determinant of a matrix is a scalar value that can be used to describe the scaling effect of the matrix on a region of space. It is a fundamental concept in mathematics and has numerous applications in various fields, including physics, engineering, and computer science.

Q: Why is the determinant of a matrix important?

A: The determinant of a matrix is important because it can be used to determine the invertibility of the matrix, the existence of solutions to a system of linear equations, and the stability of a linear system. It is also used in various applications, such as finding the area of a parallelogram, the volume of a parallelepiped, and the distance between two points in space.

Q: How do I calculate the determinant of a 3x3 matrix?

A: There are three methods for calculating the determinant of a 3x3 matrix: the expansion by minors, the cofactor expansion, and the LU decomposition. We discussed each of these methods in our previous article.

Q: What is the expansion by minors method?

A: The expansion by minors method involves expanding the determinant along a row or column of the matrix. To expand along a row, we multiply each element in the row by its minor, which is the determinant of the 2x2 matrix obtained by removing the row and column of the element. We then add or subtract the products of the elements and their minors, depending on their position in the row.

Q: What is the cofactor expansion method?

A: The cofactor expansion method involves expanding the determinant along a row or column of the matrix, using the cofactors of the elements instead of the minors. The cofactor of an element is the determinant of the 2x2 matrix obtained by removing the row and column of the element, multiplied by $ (-1)^{i+j} $, where $ i $ and $ j $ are the row and column indices of the element.

Q: What is the LU decomposition method?

A: The LU decomposition method involves decomposing the matrix into the product of a lower triangular matrix and an upper triangular matrix. The determinant of the matrix can then be calculated as the product of the diagonal elements of the lower and upper triangular matrices.

Q: What are some common mistakes to avoid when calculating the determinant of a 3x3 matrix?

A: Some common mistakes to avoid when calculating the determinant of a 3x3 matrix include:

Not expanding along a row or column correctly
Not calculating the minors or cofactors correctly
Not multiplying the elements and their minors or cofactors correctly
Not adding or subtracting the products of the elements and their minors or cofactors correctly

Q: How do I check my work when calculating the determinant of a 3x3 matrix?

A: To check your work when calculating the determinant of a 3x3 matrix, you can use the following steps:

Calculate the determinant using each of the three methods (expansion by minors, cofactor expansion, and LU decomposition)
Compare the results to ensure that they are the same
Check your work by plugging the matrix into a calculator or computer program to calculate the determinant

Conclusion

In this article, we have answered some frequently asked questions about the determinant of a 3x3 matrix. We have discussed the importance of the determinant, the three methods for calculating it, and some common mistakes to avoid. We have also provided some tips for checking your work when calculating the determinant.

References

[1] Strang, G. (1988). Linear Algebra and Its Applications. 3rd ed. New York: Harper & Row.
[2] Lay, D. C. (2005). Linear Algebra and Its Applications. 3rd ed. Boston: Addison-Wesley.
[3] Anton, H. (1994). Elementary Linear Algebra. 7th ed. New York: Wiley.