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Transcript of 3.8 Matrices. Matrix DEFINITION 1 A matrix is a rectangular array of numbers. A matrix with m rows...
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3.8 Matrices
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Matrix
DEFINITION 1• A matrix is a rectangular array of numbers.• A matrix with m rows and n columns is called
an m x n matrix.• The size of an mxn matrix is mxn • The plural of matrix is matrices.• A square matrix is a matrix with the same
number of rows and columns
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Example 1
The matrix is a 3 x 2 matrix.
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• A matrix with only one column is called a column vector or a column matrix.
Example:It is a 3x1 column matrix
• A matrix with only one raw is called a raw vector or a raw matrix.
Example:It is a 1x3 column matrix
A Column Matrix & Raw Matrix
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Main Diagonal
The shaded entries (a)11 , (a)22 , …., (a)nn are said to be on the main diagonal of A.
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Example:
(a)11 = 1, (a)12 = 1
(a)21 = 0, (a)22 = 2
(a)31 = 1, (a)32 = 3
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Equality of Tow MatricesDefinitionTwo matrices are defined to be equal if • They have the same size and• Their corresponding entries are equal.
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Example:Consider the matrices
• If x= 5, then A=B, but for all other values of x the matrices A and B are not equal, since not all of their corresponding entries are equal.
• There is no value of x for which A=C since A and C have different sizes.
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Matrix Arithmetic
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Matrix Operations
• Addition and Subtraction.• Scalar Multiples.• Multiplying Matrices.
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Addition and Subtraction of matricesDEFINITION 3• If A and B are matrices of the same size, then the
sum A+B is the matrix obtained by adding the entries of B to the corresponding entries of A, and
• The difference A-B is the matrix obtained by subtracting the entries of B from the corresponding entries of A.
• Matrices of different sizes cannot be added or subtracted.
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EXAMPLE 2Consider the matrices
Then andThe expressions A+C, B+C, A-C, and B-C are
undefined.
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Scalar Multiples.
DEFINITION 3If A is any matrix and c is any scalar, then the
product cA is the matrix obtained by multiplying each entry of the matrix A by c.
The matrix cA is said to be a scalar multiple ofA.In matrix notation,If A =[aij], then cA =c[aij], = [caij].
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Example:For the matrices
We have
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Multiplying Matrices.DEFINITION 4Let A be an m x k matrix and B be a k x n matrix.
The product of A and B, denoted by AB, isthe m x n matrix with its (i,j)th entry equal to the
sum of the products of the correspondingelements from the ith row of A and the jth
column of B. In other words, if AB = [cij] then cij = a1j b1j + ai2b2i
+ . . . + aikbki.
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Determining Whether a product Is Defined
Suppose that A, B, and C are matrices with the following sizes:
A B C 3x4 4x7 7x3Then• AB is defined and is a 3x7 matrix;• BC is defined and is a 4x3 matrix;• CA is defined and is a 7x4 matrix.• The products AC, CB, and BA are all undefined.
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EXAMPLE 3
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Coution!
AB≠BAEXAMPLE 4
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AB≠BA
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Remark
• AB may be defined but BA may not. (e.g. if A is 2x3 and B is 3x4)
• AB and BA may both defined, but they may have different sizes.(e.g. if A is 2x3 and B is 3x2)
• AB and BA may both defined and have the same sizes, but the two matrices may be different. (see the previous example).
AB≠BA
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The Identity Matrices
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DefinitionA matrix whose entries are all zero is called a
zero matrix.We will denote a zero matrix by 0 unless it is
important to specify its size, in which case we will denote the mxn zero matrix by 0mxn .
Examples:
The Zero Matrices
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Remark
• If A and 0 are matrices of the same sizes, thenA+0= 0+A= A & A-0 = 0-A = A• If A and 0 are matrices of a different sizes,
thenA+0 , 0+A, A-0 & 0-A are not defined.
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Transpose of Matrix
DEFINITION 6If A is any mxn matrix, then the
transpose of A, denoted by AT , is defined to be the nxm matrix that results by interchanging rows and columns of matrix A; that is; the first column of AT is the first row of A, and the second column of AT is the column row of A, and so forth.
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Example:The following are some examples of matrices
and their transposes.
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The Trace of a MatrixDefinitionIf A is a square matrix, then the trace of A,
denoted by tr(A), is defined to be the sum of the entries on the main diagonal of A. The trace of A is undefined if A is not a square matrix.
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Example:The following are example of matrices and
their traces:
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Singular, nonsingular, and Inverse Matrices
DefinitionIf A is a square matrix, and if a matrix B of the
same size can be found such that AB=BA=I, then A is said to be invertible or nonsingular, and B is called an inverse of A.
If no such matrix B can be found, then A is said to be singular.
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Example:Let
Then
Thus, A and B are invertible and each is an inverse of the other.
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Calculating the Inverse of a 2x2 MatrixTheoremThe matrix
is invertible if and only if ad-bc≠0, in which case the inverse is giving by the formula
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Example:In each part, determine whether the matrix is
invertible. If so, find its inverse.
Solution:(a) The determination of A is det(A)=(6)(2)-(1)(5)=7≠0. Then A is invertible, and its
inverse is
(b) det(A)=(-1)(-6)-(2)(3)=0Then A is not invertible.
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HomeworkPage 254• 1(a,b,c,d,e)• 2(a,b,c,d)• 3(a,b,c)• 20(a)Page 35• 1(a,b,c,d,e,f,g,h)• 2(a,b,c,d,e,f,g,h)• 3(a,b,c,d,e,h,i,j,k)• 17
Page 49• 1(a,d)• 3(c)• 4• 5• 12• 14• 18 (c).
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