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3.1 The Determinant of a Matrix
Note:
2221
1211
aa
aa
2221
1211
aa
aa
The determinant of a matrix can be positive, zero, or negative.
Chapter 3Determinants
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Notes: Sign pattern for cofactors
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• The determinant of a matrix of order 3:
333231
232221
131211
aaa
aaa
aaa
A
3231333231
2221232221
1211131211
aaaaa
aaaaa
aaaaa
122133112332
132231322113312312332211
||)det(
aaaaaa
aaaaaaaaaaaaAA
Add these three products.
Subtract these three products.
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• Upper triangular matrix:
Lower triangular matrix:
Diagonal matrix:
All the entries below the main diagonal are zeros.
All the entries above the main diagonal are zeros.
All the entries above and below the main diagonal are zeros.
33
2322
131211
000
aaaaaa
333231
2221
11
000
aaaaa
a Ex:
upper triangular
33
22
11
000000
aa
a
lower triangular diagonal
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• A row-echelon form of a square matrix is always upper triangular.
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3.2 Evaluation of a Determinant Using Elementary Operations
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• Determinants and Elementary Column Operations: The elementary row operations can be replaced by the column operations and two matrices are called column-equivalent if one can be obtained form the other by elementary column operations.
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3.3 Properties of Determinants
• Notes:
)det()det()det( BABA (1)
(2)
333231
232221
131211
333231
232221
131211
aaa
bbb
aaa
aaa
aaa
aaa
333231
232322222222
131211
aaa
bababa
aaa
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3.4 Applications of Determinants
Matrix of cofactors of A:
nnnn
n
n
ij
CCC
CCC
CCC
C
21
22221
11211
ijji
ij MC )1(
nnnn
n
n
Tij
CCC
CCC
CCC
CAadj
21
22212
12111
)(
Adjoint matrix of A:
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