# Chapter 3 Determinants 3.1 The Determinant of a Matrix

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Chapter 3 Determinants 3.1 The Determinant of a Matrix
3.2 Evaluation of a Determinant using Elementary Operations 3.3 Properties of Determinants 3.4 Introduction to Eigenvalues 3.5 Application of Determinants Elementary Linear Algebra 投影片設計編製者 R. Larsen et al. (6 Edition) 淡江大學 電機系 翁慶昌 教授

3.1 The Determinant of a Matrix
Note: Elementary Linear Algebra: Section 3.1, p.123

Ex. 1: (The determinant of a matrix of order 2)
Note: The determinant of a matrix can be positive, zero, or negative. Elementary Linear Algebra: Section 3.1, p.123

Minor of the entry : The determinant of the matrix determined by deleting the ith row and jth column of A Cofactor of : Elementary Linear Algebra: Section 3.1, p.124

Ex: Elementary Linear Algebra: Section 3.1, p.124

Notes: Sign pattern for cofactors
3 × 3 matrix × 4 matrix n ×n matrix Notes: Odd positions (where i+j is odd) have negative signs, and even positions (where i+j is even) have positive signs. Elementary Linear Algebra: Section 3.1, p.124

Ex 2: Find all the minors and cofactors of A.
Sol: (1) All the minors of A. Elementary Linear Algebra: Section 3.1, p.125

Sol: (2) All the cofactors of A.
Elementary Linear Algebra: Section 3.1, p.125

Thm 3.1: (Expansion by cofactors) Let A is a square matrix of order n.
Then the determinant of A is given by (Cofactor expansion along the i-th row, i=1, 2,…, n ) or (Cofactor expansion along the j-th row, j=1, 2,…, n ) Elementary Linear Algebra: Section 3.1, p.126

Ex: The determinant of a matrix of order 3
Elementary Linear Algebra: Section 3.1, Addition

Ex 3: The determinant of a matrix of order 3
Sol: Elementary Linear Algebra: Section 3.1, p.126

Ex 5: (The determinant of a matrix of order 3)
Sol: Elementary Linear Algebra: Section 3.1, p.128

The row (or column) containing the most zeros is the best choice
Notes: The row (or column) containing the most zeros is the best choice for expansion by cofactors . Ex 4: (The determinant of a matrix of order 4) Elementary Linear Algebra: Section 3.1, p.127

Sol: Elementary Linear Algebra: Section 3.1, p.127

The determinant of a matrix of order 3:
Subtract these three products. Add these three products. Elementary Linear Algebra: Section 3.1, p.127

Ex 5: –4 6 16 –12 Elementary Linear Algebra: Section 3.1, p.128

Upper triangular matrix:
All the entries below the main diagonal are zeros. Lower triangular matrix: All the entries above the main diagonal are zeros. Diagonal matrix: All the entries above and below the main diagonal are zeros. Note: A matrix that is both upper and lower triangular is called diagonal. Elementary Linear Algebra: Section 3.1, p.128

Ex: diagonal upper triangular lower triangular
Elementary Linear Algebra: Section 3.1, p.128

Thm 3.2: (Determinant of a Triangular Matrix)
If A is an n × n triangular matrix (upper triangular, lower triangular, or diagonal), then its determinant is the product of the entries on the main diagonal. That is Elementary Linear Algebra: Section 3.1, p.129

Ex 6: Find the determinants of the following triangular matrices.
(b) Sol: (a) |A| = (2)(–2)(1)(3) = –12 (b) |B| = (–1)(3)(2)(4)(–2) = 48 Elementary Linear Algebra: Section 3.1, p.129

Keywords in Section 3.1: determinant : 行列式 minor : 子行列式 cofactor : 餘因子
expansion by cofactors : 餘因子展開 upper triangular matrix: 上三角矩陣 lower triangular matrix: 下三角矩陣 diagonal matrix: 對角矩陣

3.2 Evaluation of a determinant using elementary operations
Thm 3.3: (Elementary row operations and determinants) Let A and B be square matrices. Elementary Linear Algebra: Section 3.2, p.134

Ex: Elementary Linear Algebra: Section 3.2, Addition

Notes: Elementary Linear Algebra: Section 3.2, pp

A row-echelon form of a square matrix is always upper triangular.
Note: A row-echelon form of a square matrix is always upper triangular. Ex 2: (Evaluation a determinant using elementary row operations) Sol: Elementary Linear Algebra: Section 3.2, pp

Elementary Linear Algebra: Section 3.2, pp.134-135

Notes: Elementary Linear Algebra: Section 3.2, Addition

Determinants and elementary column operations
Thm: (Elementary column operations and determinants) Let A and B be square matrices. Elementary Linear Algebra: Section 3.2, p.135

Ex: Elementary Linear Algebra: Section 3.2, p.135

Thm 3.4: (Conditions that yield a zero determinant)
If A is a square matrix and any of the following conditions is true, then det (A) = 0. (a) An entire row (or an entire column) consists of zeros. (b) Two rows (or two columns) are equal. (c) One row (or column) is a multiple of another row (or column). Elementary Linear Algebra: Section 3.2, p.136

Ex: Elementary Linear Algebra: Section 3.2, Addition

Note: Cofactor Expansion Row Reduction Order n 3 5 9 10 119 205 30 45
Additions Multiplications 3 5 9 10 119 205 30 45 3,628,799 6,235,300 285 339 Elementary Linear Algebra: Section 3.2, p.138

Ex 5: (Evaluating a determinant)
Sol: Elementary Linear Algebra: Section 3.2, p.138

Ex 6: (Evaluating a determinant)
Sol: Elementary Linear Algebra: Section 3.2, p.139

Elementary Linear Algebra: Section 3.2, p.140

3.3 Properties of Determinants
Thm 3.5: (Determinant of a matrix product) det (AB) = det (A) det (B) Notes: (1) det (EA) = det (E) det (A) (2) (3) Elementary Linear Algebra: Section 3.3, p.143

Ex 1: (The determinant of a matrix product)
Find |A|, |B|, and |AB| Sol: Elementary Linear Algebra: Section 3.3, p.143

Check: |AB| = |A| |B| Elementary Linear Algebra: Section 3.3, p.143

Thm 3.6: (Determinant of a scalar multiple of a matrix)
If A is an n × n matrix and c is a scalar, then det (cA) = cn det (A) Ex 2: Find |A|. Sol: Elementary Linear Algebra: Section 3.3, p.144

Thm 3.7: (Determinant of an invertible matrix)
A square matrix A is invertible (nonsingular) if and only if det (A)  0 Ex 3: (Classifying square matrices as singular or nonsingular) Sol: A has no inverse (it is singular). B has an inverse (it is nonsingular). Elementary Linear Algebra: Section 3.3, p.145

Thm 3.8: (Determinant of an inverse matrix)
Thm 3.9: (Determinant of a transpose) Ex 4: (a) (b) Sol: Elementary Linear Algebra: Section 3.3, pp

Equivalent conditions for a nonsingular matrix:
If A is an n × n matrix, then the following statements are equivalent. (1) A is invertible. (2) Ax = b has a unique solution for every n × 1 matrix b. (3) Ax = 0 has only the trivial solution. (4) A is row-equivalent to In (5) A can be written as the product of elementary matrices. (6) det (A)  0 Elementary Linear Algebra: Section 3.3, p.147

Ex 5: Which of the following system has a unique solution?
(b) Elementary Linear Algebra: Section 3.3, p.148

This system does not have a unique solution.
(b) This system has a unique solution. Elementary Linear Algebra: Section 3.3, p.148

3.4 Introduction to Eigenvalues
Eigenvalue problem: If A is an nn matrix, do there exist n1 nonzero matrices x such that Ax is a scalar multiple of x？ Eigenvalue and eigenvector: A：an nn matrix ：a scalar x： a n1 nonzero column matrix Eigenvalue Eigenvector (The fundamental equation for the eigenvalue problem) Elementary Linear Algebra: Section 3.4, p.152

Ex 1: (Verifying eigenvalues and eigenvectors)
Elementary Linear Algebra: Section 3.4, p.152

Given an nn matrix A, how can you find the eigenvalues and
Question: Given an nn matrix A, how can you find the eigenvalues and corresponding eigenvectors? Characteristic equation of AMnn: If has nonzero solutions iff Note: (homogeneous system) Elementary Linear Algebra: section 3.4, p.153

Ex 2: (Finding eigenvalues and eigenvectors)
Sol: Characteristic equation: Eigenvalues: Elementary Linear Algebra: Section 3.4, p.153

Elementary Linear Algebra: Section 3.4, p.153

Ex 3: (Finding eigenvalues and eigenvectors)
Sol: Characteristic equation: Elementary Linear Algebra: Section 3.4, p.154

Elementary Linear Algebra: Section 3.4, p.155

Elementary Linear Algebra: Section 3.4, p.156

3.5 Applications of Determinants
Matrix of cofactors of A: Adjoint matrix of A: Elementary Linear Algebra: Section 3.5, p.158

Thm 3.10: (The inverse of a matrix given by its adjoint)
If A is an n × n invertible matrix, then Ex: Elementary Linear Algebra: Section 3.5, p.159

(a) Find the adjoint of A.
Ex 1 & Ex 2: (a) Find the adjoint of A. (b) Use the adjoint of A to find Sol: Elementary Linear Algebra: Section 3.5, p.160

adjoint matrix of A cofactor matrix of A inverse matrix of A Check:
Elementary Linear Algebra: Section 3.5, p.160

(this system has a unique solution)
Thm 3.11: (Cramer’s Rule) (this system has a unique solution) Elementary Linear Algebra: Section 3.5, p.163

( i.e. ) Elementary Linear Algebra: Section 3.5, p.163

Pf: A x = b, Elementary Linear Algebra: Section 3.5, p.163

Elementary Linear Algebra: Section 3.5, p.163

Ex 4: Use Cramer’s rule to solve the system of linear equations.
Elementary Linear Algebra: Section 3.5, p.163

Keywords in Section 3.5: matrix of cofactors : 餘因子矩陣
adjoint matrix : 伴隨矩陣 Cramer’s rule : Cramer 法則