CHAPTER 1 إعداد د. هيله العودان (وضعت باذن معدة العرض)

OPERATIONS ON MATRICES Definition. If A is an m × r matrix and B is an r × n matrix, then the product AB is the m × n matrix whose entries are determined as follows. To find the entry in row i and column j of AB, single out row i from the matrix B. Multiply the corresponding entries from the row and column together and then add up the resulting products.

OPERATIONS ON MATRICES Example 5 Consider the matrices A = B =

OPERATIONS ON MATRICES Since A is a 2 × 3 matrix and B is a 3 × 4 matrix, the product AB is a 2 × 4 matrix. To determine, for example, the entry in row2 and column 3 of AB, we single out row 2 from A and column 3 from B. then, as illustrated below, we multiply corresponding entries together and add up these products.

OPERATIONS ON MATRICES = (2 · 4) + (6 · 3) + (0 · 5) = 26

OPERATIONS ON MATRICES The entry in row 1 and column 4 of AB is computed as follows: = (1 · 3) + (2 · 1) + (4 · 2) = 13

OPERATIONS ON MATRICES The computations for the remaining products are (1 · 4) + (2 · 0) + (4 · 2) = 12 (1 · 1) - (2 · 1) + (4 · 7) = 27 (1 · 4) + (2 · 3) + (4 · 5) = 30 (2 · 4) + (6 · 0) + (0 · 2) = 8 (2 · 1) - (6 · 1) + (0 · 7) = - 4 (2 · 3) + (6 · 1) + (0 · 2) = 12

OPERATIONS ON MATRICES AB =

OPERATIONS ON MATRICES AB= AB m × r r × n m × n Inside Outside

OPERATIONS ON MATRICES Example 6 Suppose that A, B and C are matrices with the following sizes: ABC 3 × 4 4 × 7 7 × 3 Then AB is defined and is a 3 × 7 matrix; CA is defined and is a 7 × 4 matrix; and BC is defined and is a 4 × 3 matrix. The products AC, CB, and BA are all undefined.

MATRIX MULTIPLICATION BY COLUMNS AND BY ROWS j th column matrix of AB = A [ j th column matrix of B ] i th row matrix of AB = [ i th row matrix of A ] B

MATRIX MULTIPLICATION BY COLUMNS AND BY ROWS Example 7 if A and B are the matrices in Example 5, then from (3) the second column matrix of AB can be obtained by the computation = Second column of B Second column of AB

MATRIX MULTIPLICATION BY COLUMNS AND BY ROWS and from (4) the first row matrix of AB can be obtained by the computation = First row of A First row of AB

A x = b A is called coefficient matrix of the system

Definition. If A is any m × n matrix, then the transpose of A, denoted by A, is defined to be the n × m matrix that results from interchanging the rows and columns of A ; that is, the first column of A is the first row of A, the second column of A is the second row of A, and so forth. T T T

Example 10 The following are some examples of matrices and their transposes. A =B =C =D = TTTT a 11 a 12 a 13 a 14 a 21 a 22 a 23 a 24 a 31 a 32 a 33 a 34 a 11 a 21 a 31 a 12 a 22 a 32 a 13 a 23 a 33 a 14 a 24 a

(A ) i j = (A) j i T

A = T

Theorem Assuming that sizes of the matrices are such that the indicated operations can be performed, the following rules of matrix arithmetic are valid. (a)A+B=B+A (Commutative law for addition) (b)A+(B+C)=(A+B)+C (Associative law for addition) (c)A(BC)=(AB)C (Associative law for multiplication) (d)A(B+C)=AB+AC (Left distributive law) (e)(B+C)A=BA+CA (Right distributive law) (f)A(B-C)=AB-AC(j) (a+b)C=aC+bC (g)(B-C)A=BA-CA(k) (a-b)C=aC-bC (h)a(B+C)=aB-aC(l) a(bC)=(ab)C (i)a(B-C)=aB-aC(m) a(BC)=(aB)C=B(aC)

ZERO MATRICES A matrix, all of whose entries are zero