Chapter 3: Matrices PDF

34 MATHEMATICS Chapter 3 MATRICES v The essence of Mathematics lies in its freedom. — CANTOR v 3.1 Introduction The knowledge of matrices is necessary in various branches of mathema...

34 MATHEMATICS Chapter 3 MATRICES v The essence of Mathematics lies in its freedom. — CANTOR v 3.1 Introduction The knowledge of matrices is necessary in various branches of mathematics. Matrices are one of the most powerful tools in mathematics. This mathematical tool simplifies our work to a great extent when compared with other straight forward methods. The evolution of concept of matrices is the result of an attempt to obtain compact and simple methods of solving system of linear equations. Matrices are not only used as a representation of the coefficients in system of linear equations, but utility of matrices far exceeds that use. Matrix notation and operations are used in electronic spreadsheet programs for personal computer, which in turn is used in different areas of business and science like budgeting, sales projection, cost estimation, analysing the results of an experiment etc. Also, many physical operations such as magnification, rotation and reflection through a plane can be represented mathematically by matrices. Matrices are also used in cryptography. This mathematical tool is not only used in certain branches of sciences, but also in genetics, economics, sociology, modern psychology and industrial management. In this chapter, we shall find it interesting to become acquainted with the fundamentals of matrix and matrix algebra. 3.2 Matrix Suppose we wish to express the information that Radha has 15 notebooks. We may express it as with the understanding that the number inside [ ] is the number of notebooks that Radha has. Now, if we have to express that Radha has 15 notebooks and 6 pens. We may express it as [15 6] with the understanding that first number inside [ ] is the number of notebooks while the other one is the number of pens possessed by Radha. Let us now suppose that we wish to express the information of possession Reprint 2024-25  MATRICES 35 of notebooks and pens by Radha and her two friends Fauzia and Simran which is as follows: Radha has 15 notebooks and 6 pens, Fauzia has 10 notebooks and 2 pens, Simran has 13 notebooks and 5 pens. Now this could be arranged in the tabular form as follows: Notebooks Pens Radha 15 6 Fauzia 10 2 Simran 13 5 and this can be expressed as or Radha Fauzia Simran Notebooks 15 10 13 Pens 6 2 5 which can be expressed as: In the first arrangement the entries in the first column represent the number of note books possessed by Radha, Fauzia and Simran, respectively and the entries in the second column represent the number of pens possessed by Radha, Fauzia and Simran, Reprint 2024-25 36 MATHEMATICS respectively. Similarly, in the second arrangement, the entries in the first row represent the number of notebooks possessed by Radha, Fauzia and Simran, respectively. The entries in the second row represent the number of pens possessed by Radha, Fauzia and Simran, respectively. An arrangement or display of the above kind is called a matrix. Formally, we define matrix as: Definition 1 A matrix is an ordered rectangular array of numbers or functions. The numbers or functions are called the elements or the entries of the matrix. We denote matrices by capital letters. The following are some examples of matrices:  1 – 2 5  2 + i 3 − 2      1 + x x3 3  A= 0 5  , B =  3.5 –1 2  , C =      cos x sin x + 2 tan x  3 6  5   3 5   7  In the above examples, the horizontal lines of elements are said to constitute, rows of the matrix and the vertical lines of elements are said to constitute, columns of the matrix. Thus A has 3 rows and 2 columns, B has 3 rows and 3 columns while C has 2 rows and 3 columns. 3.2.1 Order of a matrix A matrix having m rows and n columns is called a matrix of order m × n or simply m × n matrix (read as an m by n matrix). So referring to the above examples of matrices, we have A as 3 × 2 matrix, B as 3 × 3 matrix and C as 2 × 3 matrix. We observe that A has 3 × 2 = 6 elements, B and C have 9 and 6 elements, respectively. In general, an m × n matrix has the following rectangular array: or A = [aij]m × n, 1≤ i ≤ m, 1≤ j ≤ n i, j ∈ N Thus the ith row consists of the elements ai1, ai2, ai3,..., ain, while the jth column consists of the elements a1j, a2j, a3j,..., amj , In general aij, is an element lying in the ith row and jth column. We can also call it as the (i, j)th element of A. The number of elements in an m × n matrix will be equal to mn. Reprint 2024-25  MATRICES 37 ANote In this chapter 1. We shall follow the notation, namely A = [aij]m × n to indicate that A is a matrix of order m × n. 2. We shall consider only those matrices whose elements are real numbers or functions taking real values. We can also represent any point (x, y) in a plane by a matrix (column or row) as  x  y  (or [x, y]). For example point P(0, 1) as a matrix representation may be given as   0 P =   or [0 1]. 1  Observe that in this way we can also express the vertices of a closed rectilinear figure in the form of a matrix. For example, consider a quadrilateral ABCD with vertices A (1, 0), B (3, 2), C (1, 3), D (–1, 2). Now, quadrilateral ABCD in the matrix form, can be represented as A B C D A 1 0 1 3 1 −1 B  3 2  X=  or Y=  0 2 3 2 2 × 4 C 1 3   D −1 2  4× 2 Thus, matrices can be used as representation of vertices of geometrical figures in a plane. Now, let us consider some examples. Example 1 Consider the following information regarding the number of men and women workers in three factories I, II and III Men workers Women workers I 30 25 II 25 31 III 27 26 Represent the above information in the form of a 3 × 2 matrix. What does the entry in the third row and second column represent? Reprint 2024-25 38 MATHEMATICS Solution The information is represented in the form of a 3 × 2 matrix as follows: 30 25 A =  25 31 27 26  The entry in the third row and second column represents the number of women workers in factory III. Example 2 If a matrix has 8 elements, what are the possible orders it can have? Solution We know that if a matrix is of order m × n, it has mn elements. Thus, to find all possible orders of a matrix with 8 elements, we will find all ordered pairs of natural numbers, whose product is 8. Thus, all possible ordered pairs are (1, 8), (8, 1), (4, 2), (2, 4) Hence, possible orders are 1 × 8, 8 ×1, 4 × 2, 2 × 4 1 Example 3 Construct a 3 × 2 matrix whose elements are given by aij = |i −3j |. 2  a11 a12  Solution In general a 3 × 2 matrix is given by A =  a21 a22 .    a31 a32  1 Now aij = | i − 3 j | , i = 1, 2, 3 and j = 1, 2. 2 1 1 5 Therefore a11 = |1 − 3 × 1| = 1 a12 = |1 − 3 × 2 | = 2 2 2 1 1 1 a21 = | 2 − 3 × 1| = a22 = | 2 − 3× 2 | = 2 2 2 2 1 1 3 a31 = | 3 − 3 × 1| = 0 a32 = | 3 − 3× 2 | = 2 2 2 1 5  2 1  Hence the required matrix is given by A =  2. 2 3 0   2 Reprint 2024-25  MATRICES 39 3.3 Types of Matrices In this section, we shall discuss different types of matrices. (i) Column matrix A matrix is said to be a column matrix if it has only one column.  0     3 For example, A =  −1  is a column matrix of order 4 × 1.   1/ 2  In general, A = [aij] m × 1 is a column matrix of order m × 1. (ii) Row matrix A matrix is said to be a row matrix if it has only one row.  1 For example, B = −  5 2 3 is a row matrix.  2 1× 4 In general, B = [bij] 1 × n is a row matrix of order 1 × n. (iii) Square matrix A matrix in which the number of rows are equal to the number of columns, is said to be a square matrix. Thus an m × n matrix is said to be a square matrix if m = n and is known as a square matrix of order ‘n’.  3 −1 0 3  For example A =  3 2 1  is a square matrix of order 3. 2  4 3 −1  In general, A = [aij] m × m is a square matrix of order m. ANote If A = [a ] is a square matrix of order n, then elements (entries) a , a ,..., a ij 11 22 nn 1 −3 1    are said to constitute the diagonal, of the matrix A. Thus, if A = 2 4 −1. 3 5 6  Then the elements of the diagonal of A are 1, 4, 6. Reprint 2024-25 40 MATHEMATICS (iv) Diagonal matrix A square matrix B = [bij] m × m is said to be a diagonal matrix if all its non diagonal elements are zero, that is a matrix B = [bij] m × m is said to be a diagonal matrix if bij = 0, when i ≠ j.  −1.1 0 0  −1 0   For example, A = , B =  , C= 0 2 0 , are diagonal matrices  0 2   0 0 3 of order 1, 2, 3, respectively. (v) Scalar matrix A diagonal matrix is said to be a scalar matrix if its diagonal elements are equal, that is, a square matrix B = [bij] n × n is said to be a scalar matrix if bij = 0, when i ≠ j bij = k, when i = j, for some constant k. For example  3 0 0  −1 0    A = , B= , C= 0 3 0  0 −1   0 0 3 are scalar matrices of order 1, 2 and 3, respectively. (vi) Identity matrix A square matrix in which elements in the diagonal are all 1 and rest are all zero is called an identity matrix. In other words, the square matrix A = [aij] n × n is an 1 if i = j identity matrix, if aij = . 0 if i ≠ j We denote the identity matrix of order n by In. When order is clear from the context, we simply write it as I. 1 0 0  1 0  0 1 0  For example ,  ,   are identity matrices of order 1, 2 and 3, 0 1  0 0 1    respectively. Observe that a scalar matrix is an identity matrix when k = 1. But every identity matrix is clearly a scalar matrix. Reprint 2024-25  MATRICES 41 (vii) Zero matrix A matrix is said to be zero matrix or null matrix if all its elements are zero. 0 0  0 0 0  For example, ,  ,   , [0, 0] are all zero matrices. We denote 0 0  0 0 0  zero matrix by O. Its order will be clear from the context. 3.3.1 Equality of matrices Definition 2 Two matrices A = [aij] and B = [bij] are said to be equal if (i) they are of the same order (ii) each element of A is equal to the corresponding element of B, that is aij = bij for all i and j.  2 3  2 3 3 2  2 3 For example,   and   are equal matrices but   and   are  0 1 0 1 0 1  0 1  not equal matrices. Symbolically, if two matrices A and B are equal, we write A = B.  x y   −1.5 0    If  z a  =  2 6  , then x = – 1.5, y = 0, z = 2, a = 6 , b = 3, c = 2   b c  3 2   x + 3 z + 4 2 y − 7  0 6 3y − 2   −6 a −1   0  = − 6 −3 2c + 2 Example 4 If  b − 3 − 21 0   2b + 4 − 21 0  Find the values of a, b, c, x, y and z. Solution As the given matrices are equal, therefore, their corresponding elements must be equal. Comparing the corresponding elements, we get x + 3 = 0, z + 4 = 6, 2y – 7 = 3y – 2 a – 1 = – 3, 0 = 2c + 2 b – 3 = 2b + 4, Simplifying, we get a = – 2, b = – 7, c = – 1, x = – 3, y = –5, z = 2 Example 5 Find the values of a, b, c, and d from the following equation:  2a + b a − 2b   4 −3  5c − d 4c + 3d  = 11 24      Reprint 2024-25 42 MATHEMATICS Solution By equality of two matrices, equating the corresponding elements, we get 2a + b = 4 5c – d = 11 a – 2b = – 3 4c + 3d = 24 Solving these equations, we get a = 1, b = 2, c = 3 and d = 4 EXERCISE 3.1 2 5 19 −7   5  1. In the matrix A =  35 −2 12  , write:  2     3 1 −5 17  (i) The order of the matrix, (ii) The number of elements, (iii) Write the elements a13, a21, a33, a24, a23. 2. If a matrix has 24 elements, what are the possible orders it can have? What, if it has 13 elements? 3. If a matrix has 18 elements, what are the possible orders it can have? What, if it has 5 elements? 4. Construct a 2 × 2 matrix, A = [aij], whose elements are given by: (i + j ) 2 i (i + 2 j )2 (i) aij = (ii) aij = (iii) aij = 2 j 2 5. Construct a 3 × 4 matrix, whose elements are given by: 1 (i) aij = | −3i + j | (ii) aij = 2i − j 2 6. Find the values of x, y and z from the following equations:  x + y + z  9   4 3  y z  x + y 2  6 2   x + z  = 5 (i)  =  (ii)  = (iii)  x 5  1 5 5 + z xy   5 8       y + z  7  7. Find the value of a, b, c and d from the equation:  a − b 2a + c   −1 5   2a − b 3c + d  =  0 13     Reprint 2024-25  MATRICES 43 8. A = [aij]m × n\ is a square matrix, if (A) m < n (B) m > n (C) m = n (D) None of these 9. Which of the given values of x and y make the following pair of matrices equal 3x + 7 5  0 y − 2   y + 1 2 − 3 x  , 8 4     −1 (A) x = , y =7 (B) Not possible to find 3 −2 −1 −2 (C) y = 7, x= (D) x = , y= 3 3 3 10. The number of all possible matrices of order 3 × 3 with each entry 0 or 1 is: (A) 27 (B) 18 (C) 81 (D) 512 3.4 Operations on Matrices In this section, we shall introduce certain operations on matrices, namely, addition of matrices, multiplication of a matrix by a scalar, difference and multiplication of matrices. 3.4.1 Addition of matrices Suppose Fatima has two factories at places A and B. Each factory produces sport shoes for boys and girls in three different price categories labelled 1, 2 and 3. The quantities produced by each factory are represented as matrices given below: Suppose Fatima wants to know the total production of sport shoes in each price category. Then the total production In category 1 : for boys (80 + 90), for girls (60 + 50) In category 2 : for boys (75 + 70), for girls (65 + 55) In category 3 : for boys (90 + 75), for girls (85 + 75) 80 + 90 60 + 50 This can be represented in the matrix form as 75 + 70 65 + 55.  90 + 75 85 + 75  Reprint 2024-25 44 MATHEMATICS This new matrix is the sum of the above two matrices. We observe that the sum of two matrices is a matrix obtained by adding the corresponding elements of the given matrices. Furthermore, the two matrices have to be of the same order. a11 a12 a13  b11 b12 b13  Thus, if A =   is a 2 × 3 matrix and B =   is another a21 a22 a23  b21 b22 b23   a11 + b11 a12 + b12 a13 + b13  2×3 matrix. Then, we define A + B =  .  a21 + b21 a22 + b22 a23 + b23  In general, if A = [aij] and B = [bij] are two matrices of the same order, say m × n. Then, the sum of the two matrices A and B is defined as a matrix C = [cij]m × n, where cij = aij + bij, for all possible values of i and j.  3 1 − 1 2 5 1 Example 6 Given A =     and B =  1  , find A + B  2 3 0  −2 3  2  Since A, B are of the same order 2 × 3. Therefore, addition of A and B is defined and is given by 2 + 3 1 + 5 1 − 1  2 + 3 1 + 5 0  A+B =    1 =  1  2 − 2 3+3 0+ 0 6  2  2  A Note 1. We emphasise that if A and B are not of the same order, then A + B is not 2 3 1 2 3 defined. For example if A =   , B=  , then A + B is not defined. 1 0 1 0 1  2. We may observe that addition of matrices is an example of binary operation on the set of matrices of the same order. 3.4.2 Multiplication of a matrix by a scalar Now suppose that Fatima has doubled the production at a factory A in all categories (refer to 3.4.1). Reprint 2024-25  MATRICES 45 Previously quantities (in standard units) produced by factory A were Revised quantities produced by factory A are as given below: Boys Girls 1  2 × 80 2 × 60  2  2 × 75 2 × 65 3  2 × 90 2 × 85  160 120  This can be represented in the matrix form as 150 130 . We observe that  180 170  the new matrix is obtained by multiplying each element of the previous matrix by 2. In general, we may define multiplication of a matrix by a scalar as follows: if A = [aij] m × n is a matrix and k is a scalar, then kA is another matrix which is obtained by multiplying each element of A by the scalar k. In other words, kA = k [aij] m × n = [k (aij)] m × n, that is, (i, j)th element of kA is kaij for all possible values of i and j.  3 1 1.5   For example, if A =  5 7 −3  , then 2 0 5    3 1 1.5  9 3 4.5     3A = 3  5 7 −3  = 3 5 21 −9  2 0 5  6 0 15     Negative of a matrix The negative of a matrix is denoted by – A. We define – A = (– 1) A. Reprint 2024-25 46 MATHEMATICS  3 1 For example, let A=   , then – A is given by  −5 x   3 1   −3 −1  – A = (– 1) A = (−1)  =   −5 x   5 − x  Difference of matrices If A = [aij], B = [bij] are two matrices of the same order, say m × n, then difference A – B is defined as a matrix D = [dij], where dij = aij – bij, for all value of i and j. In other words, D = A – B = A + (–1) B, that is sum of the matrix A and the matrix – B. 1 2 3  3 −1 3  Example 7 If A =   and B =   , then find 2A – B.  2 3 1  −1 0 2  Solution We have 1 2 3  3 −1 3  2A – B = 2  −   2 3 1  −1 0 2  2 4 6  −3 1 −3 =  +   4 6 2   1 0 −2   2 − 3 4 + 1 6 − 3   −1 5 3  =  =  4 + 1 6 + 0 2 − 2  5 6 0 3.4.3 Properties of matrix addition The addition of matrices satisfy the following properties: (i) Commutative Law If A = [aij], B = [bij] are matrices of the same order, say m × n, then A + B = B + A. Now A + B = [aij] + [bij] = [aij + bij] = [bij + aij] (addition of numbers is commutative) = ([bij] + [aij]) = B + A (ii) Associative Law For any three matrices A = [aij], B = [bij], C = [cij] of the same order, say m × n, (A + B) + C = A + (B + C). Now (A + B) + C = ([aij] + [bij]) + [cij] = [aij + bij] + [cij] = [(aij + bij) + cij] = [aij + (bij + cij)] (Why?) = [aij] + [(bij + cij)] = [aij] + ([bij] + [cij]) = A + (B + C) Reprint 2024-25  MATRICES 47 (iii) Existence of additive identity Let A = [a ij] be an m × n matrix and O be an m × n zero matrix, then A + O = O + A = A. In other words, O is the additive identity for matrix addition. (iv) The existence of additive inverse Let A = [aij]m × n be any matrix, then we have another matrix as – A = [– aij]m × n such that A + (– A) = (– A) + A= O. So – A is the additive inverse of A or negative of A. 3.4.4 Properties of scalar multiplication of a matrix If A = [aij] and B = [bij] be two matrices of the same order, say m × n, and k and l are scalars, then (i) k(A +B) = k A + kB, (ii) (k + l)A = k A + l A (ii) k (A + B) = k ([aij] + [bij]) = k [aij + bij] = [k (aij + bij)] = [(k aij) + (k bij)] = [k aij] + [k bij] = k [aij] + k [bij] = kA + kB (iii) ( k + l) A = (k + l) [aij] = [(k + l) aij] + [k aij] + [l aij] = k [aij] + l [aij] = k A + l A 8 0   2 −2     Example 8 If A = 4 −2 and B =  4 2  , then find the matrix X, such that 3 6  −5 1  2A + 3X = 5B. Solution We have 2A + 3X = 5B or 2A + 3X – 2A = 5B – 2A or 2A – 2A + 3X = 5B – 2A (Matrix addition is commutative) or O + 3X = 5B – 2A (– 2A is the additive inverse of 2A) or 3X = 5B – 2A (O is the additive identity) 1 or X= (5B – 2A) 3   2 −2  8 0     10 −10   −16 0   1      1    −8 4   or X =  5  4 2  − 2  4 −2   =   20 10  +   3 3   −25 5    −5 1   3 6      −6 −12   Reprint 2024-25 48 MATHEMATICS  −10   −2 3   10 − 16 −10 + 0  − 6 −10    1  1   4 14  =  20 − 8 10 + 4  = 3 12 14  =  3 3   −25 − 6 5 − 12   −31 −7     −31 −7   3 3  5 2 3 6  Example 9 Find X and Y, if X + Y =   and X − Y =  . 0 9   0 −1 5 2  3 6  Solution We have ( X + Y ) + ( X − Y ) =  + .  0 9   0 −1 8 8 8 8  or (X + X) + (Y – Y) =   ⇒ 2X =   0 8 0 8  1 8 8 4 4  or X= 0 8 = 0 4  2     5 2   3 6  Also (X + Y) – (X – Y) =  −   0 9   0 −1 5 − 3 2 − 6   2 −4  or (X – X) + (Y + Y) =   ⇒ 2Y =    0 9 +1  0 10  1  2 − 4   1 −2  or Y=  0 10 =  0 5  2     Example 10 Find the values of x and y from the following equation: x 5  3 −4  7 6 2  +  =   7 y − 3 1 2  15 14 Solution We have x 5   3 −4  7 6 2x 10   3 − 4   7 6  2 + ⇒  7 y − 3 1 2  =   + = 2  15 14 15 14 14 2 y − 6 1 Reprint 2024-25  MATRICES 49 2x + 3 10 − 4  7 6 2x + 3 6  7 6 or  14 + 1 2 y − 6 + 2  =  ⇒  =    15 14   15 2 y − 4 15 14  or 2x + 3 = 7 and 2y – 4 = 14 (Why?) or 2x = 7 – 3 and 2y = 18 4 18 or x= and y= 2 2 i.e. x =2 and y = 9. Example 11 Two farmers Ramkishan and Gurcharan Singh cultivates only three varieties of rice namely Basmati, Permal and Naura. The sale (in Rupees) of these varieties of rice by both the farmers in the month of September and October are given by the following matrices A and B. (i) Find the combined sales in September and October for each farmer in each variety. (ii) Find the decrease in sales from September to October. (iii) If both farmers receive 2% profit on gross sales, compute the profit for each farmer and for each variety sold in October. Solution (i) Combined sales in September and October for each farmer in each variety is given by Reprint 2024-25 50 MATHEMATICS (ii) Change in sales from September to October is given by 2 (iii) 2% of B = × B = 0.02 × B 100 = 0.02 = Thus, in October Ramkishan receives ` 100, ` 200 and ` 120 as profit in the sale of each variety of rice, respectively, and Grucharan Singh receives profit of ` 400, ` 200 and ` 200 in the sale of each variety of rice, respectively. 3.4.5 Multiplication of matrices Suppose Meera and Nadeem are two friends. Meera wants to buy 2 pens and 5 story books, while Nadeem needs 8 pens and 10 story books. They both go to a shop to enquire about the rates which are quoted as follows: Pen – ` 5 each, story book – ` 50 each. How much money does each need to spend? Clearly, Meera needs ` (5 × 2 + 50 × 5) that is ` 260, while Nadeem needs (8 × 5 + 50 × 10) `, that is ` 540. In terms of matrix representation, we can write the above information as follows: Requirements Prices per piece (in Rupees) Money needed (in Rupees) 2 5  5  5 × 2 + 5 × 50   260   8 10   50  8 × 5 + 10 × 50 =  540          Suppose that they enquire about the rates from another shop, quoted as follows: pen – ` 4 each, story book – ` 40 each. Now, the money required by Meera and Nadeem to make purchases will be respectively ` (4 × 2 + 40 × 5) = ` 208 and ` (8 × 4 + 10 × 40) = ` 432 Reprint 2024-25  MATRICES 51 Again, the above information can be represented as follows: Requirements Prices per piece (in Rupees) Money needed (in Rupees) 2 5  4  4 × 2 + 40 × 5   208  8 10   40   8 × 4 + 10 × 4 0 =  432          Now, the information in both the cases can be combined and expressed in terms of matrices as follows: Requirements Prices per piece (in Rupees) Money needed (in Rupees) 2 5  5 4  5 × 2 + 5 × 50 4 × 2 + 40 × 5   8 10  50 40  8 × 5 + 10 × 5 0 8 × 4 + 10 × 4 0        260 208  =    540 432  The above is an example of multiplication of matrices. We observe that, for multiplication of two matrices A and B, the number of columns in A should be equal to the number of rows in B. Furthermore for getting the elements of the product matrix, we take rows of A and columns of B, multiply them element-wise and take the sum. Formally, we define multiplication of matrices as follows: The product of two matrices A and B is defined if the number of columns of A is equal to the number of rows of B. Let A = [aij] be an m × n matrix and B = [bjk] be an n × p matrix. Then the product of the matrices A and B is the matrix C of order m × p. To get the (i, k)th element cik of the matrix C, we take the ith row of A and kth column of B, multiply them elementwise and take the sum of all these products. In other words, if A = [aij]m × n, B = [bjk]n × p, then the ith row of A is [ai1 ai2... ain] and the kth column of  b1k  b  n  2k  B is ..  , then cik = ai1 b1k + ai2 b2k + ai3 b3k +... + ain bnk = ∑ aij b jk. j =1 .  b   nk  The matrix C = [cik]m × p is the product of A and B.  2 7 1 −1 2  −1 1  , then the product CD is defined For example, if C =   and D =   0 3 4   5 − 4 Reprint 2024-25 52 MATHEMATICS  2 7 1 −1 2   and is given by CD =    −1 1 . This is a 2 × 2 matrix in which each  0 3 4    5 − 4 entry is the sum of the products across some row of C with the corresponding entries down some column of D. These four computations are 13 −2  Thus CD =   17 −13 6 9 2 6 0  Example 12 Find AB, if A =   and B =  . 2 3 7 9 8  Solution The matrix A has 2 columns which is equal to the number of rows of B. Hence AB is defined. Now  6(2) + 9(7) 6(6) + 9(9) 6(0) + 9(8)  AB =    2(2) + 3(7) 2(6) + 3(9) 2(0) + 3(8)  12 + 63 36 + 81 0 + 72  75 117 72  =  =    4 + 21 12 + 27 0 + 24  25 39 24  Reprint 2024-25  MATRICES 53 Remark If AB is defined, then BA need not be defined. In the above example, AB is defined but BA is not defined because B has 3 column while A has only 2 (and not 3) rows. If A, B are, respectively m × n, k × l matrices, then both AB and BA are defined if and only if n = k and l = m. In particular, if both A and B are square matrices of the same order, then both AB and BA are defined. Non-commutativity of multiplication of matrices Now, we shall see by an example that even if AB and BA are both defined, it is not necessary that AB = BA.  2 3  1 −2 3 Example 13 If A =   and B =  4 5 , then find AB, BA. Show that − 4 2 5   2 1 AB ≠ BA. Solution Since A is a 2 × 3 matrix and B is 3 × 2 matrix. Hence AB and BA are both defined and are matrices of order 2 × 2 and 3 × 3, respectively. Note that 2 3  1 −2 3 4  2−8+6 3 − 10 + 3   0 − 4  AB =  5 =  = − 4 2 5    −8 + 8 + 10 −12 + 10 + 5 10 3   2 1  2 3  2 − 12 − 4 + 6 6 + 15  −10 2 21  1 −2 3     and BA =  4  5   =  4 − 20 −8 + 10 12 + 25 = −16 2 37  −4 2 5   2 1   2 − 4 − 4 + 2 6 + 5   −2 −2 11  Clearly AB ≠ BA In the above example both AB and BA are of different order and so AB ≠ BA. But one may think that perhaps AB and BA could be the same if they were of the same order. But it is not so, here we give an example to show that even if AB and BA are of same order they may not be same. 1 0  0 1   0 1 Example 14 If A =   and B =   , then AB =  .  0 −1 1 0   −1 0   0 −1 and BA =  . Clearly AB ≠ BA. 1 0  Thus matrix multiplication is not commutative. Reprint 2024-25 54 MATHEMATICS ANote This does not mean that AB ≠ BA for every pair of matrices A, B for which AB and BA, are defined. For instance, 1 0  3 0  3 0 If A =   , B=  , then AB = BA =   0 2  0 4 0 8  Observe that multiplication of diagonal matrices of same order will be commutative. Zero matrix as the product of two non zero matrices We know that, for real numbers a, b if ab = 0, then either a = 0 or b = 0. This need not be true for matrices, we will observe this through an example.  0 −1 3 5  Example 15 Find AB, if A =   and B =  . 0 2  0 0   0 −1   3 5   0 0  Solution We have AB =   = . 0 2  0 0  0 0  Thus, if the product of two matrices is a zero matrix, it is not necessary that one of the matrices is a zero matrix. 3.4.6 Properties of multiplication of matrices The multiplication of matrices possesses the following properties, which we state without proof. 1. The associative law For any three matrices A, B and C. We have (AB) C = A (BC), whenever both sides of the equality are defined. 2. The distributive law For three matrices A, B and C. (i) A (B+C) = AB + AC (ii) (A+B) C = AC + BC, whenever both sides of equality are defined. 3. The existence of multiplicative identity For every square matrix A, there exist an identity matrix of same order such that IA = AI = A. Now, we shall verify these properties by examples. 1 1 −1  1 3  1 2 3 − 4 Example 16 If A =  2 0 3  , B =  0 2  and C =     , find  3 −1 2  −1 4  2 0 −2 1  A(BC), (AB)C and show that (AB)C = A(BC). Reprint 2024-25  MATRICES 55 1 1 −1  1 3   1 + 0 + 1 3 + 2 − 4   2 1  Solution We have AB = 2 0 3   0 2  =  2 + 0 − 3 6 + 0 + 12  = −1 18  3 −1 2  −1 4  3 + 0 − 2 9 − 2 + 8   1 15 2 1  2+ 2 4 + 0 6 − 2 − 8 +1  1 2 3 − 4    (AB) (C) = −1 18     2 0 −2 1 =  −1 + 36 −2 + 0 −3 − 36 4 + 18  1 15     1 + 30 2 + 0 3 − 30 − 4 + 15 4 4 4 −7  35 −2 −39 22  =   31 2 −27 11  1 3  1 + 6 2 + 0 3 − 6 −4 + 3  1 2 3 −4    Now BC =  0  2   =  0 + 4 0 + 0 0 − 4 0 + 2  2 0 −2 1  −1    4   −1 + 8 −2 + 0 −3 − 8 4 + 4  7 2 −3 −1    =  4 0 −4 2  7 −2 −11 8  1 1 −1  7 2 −3 −1  Therefore A(BC) =  2 0 3   4 0 −4 2   3 −1 2  7 −2 −11 8   7 + 4 − 7 2 + 0 + 2 −3 − 4 + 11 −1 + 2 − 8    = 14 + 0 + 21 4 + 0 − 6 −6 + 0 − 33 −2 + 0 + 24   21 − 4 + 14 6 + 0 − 4 −9 + 4 − 22 −3 − 2 + 16  4 4 4 −7  35 −2 −39 22  =  . Clearly, (AB) C = A (BC) 31 2 −27 11 Reprint 2024-25 56 MATHEMATICS  0 6 7 0 1 1  2 Example 17 If A = − 6 0 8 , B = 1 0 2 , C = −2             7 − 8 0  1 2 0   3  Calculate AC, BC and (A + B)C. Also, verify that (A + B)C = AC + BC  0 7 8   Solution Now, A + B = −5 0 10   8 − 6 0   0 7 8 2   0 − 14 + 24  10   10  −2  = −10 + 0 + 30  =  20 So (A + B) C = −5 0        8 − 6 0   3   16 + 12 + 0   28  0 6 7   2   0 − 12 + 21  9       = 12  Further AC =  −6 0 8  −2  = −12 + 0 + 24     7 − 8 0   3   14 + 16 + 0  30 0 1 1   2   0 − 2 + 3  1      =  2 + 0 + 6 =  8  and BC = 1 0 2  −2      1 2 0   3   2 − 4 + 0 − 2  9   1  10        So AC + BC = 12  +  8  = 20 30  −2   28 Clearly, (A + B) C = AC + BC 1 2 3  1 , then show that A3 – 23A – 40 I = O Example 18 If A =  3 −2  4 2 1 1 2 3 1 2 3 19 4 8 Solution We have A = A.A = 3 −2 2 1  3 −2 1 = 1 12 8   4 2 1  4 2 1 14 6 15  Reprint 2024-25  MATRICES 57 1 2 3 19 4 8   63 46 69 So A = A A =  3 −2 3 2 1 1 12 8  = 69 −6 23   4 2 1 14 6 15  92 46 63 Now  63 46 69 1 2 3 1 0 0  3 69 −6 23 – 23  3 −2 1 – 40 0 1 0   A – 23A – 40I =    92 46 63  4 2 1 0 0 1   63 46 69   −23 −46 −69  −40 0 0  69 −6 23 + −69 46 −23 +  0 −40 0  =      92 46 63 −92 −46 −23  0 0 −40  63 − 23 − 40 46 − 46 + 0 69 − 69 + 0    = 69 − 69 + 0 −6 + 46 − 40 23 − 23 + 0   92 − 92 + 0 46 − 46 + 0 63 − 23 − 40  0 0 0   =  0 0 0 = O 0 0 0 Example 19 In a legislative assembly election, a political group hired a public relations firm to promote its candidate in three ways: telephone, house calls, and letters. The cost per contact (in paise) is given in matrix A as Cost per contact  40  Telephone  100  Housecall A=    50  Letter The number of contacts of each type made in two cities X and Y is given by Telephone Housecall Letter 1000 500 5000  → X B= . Find the total amount spent by the group in the two 3000 1000 10,000 → Y cities X and Y. Reprint 2024-25 58 MATHEMATICS Solution We have  40,000 + 50,000 + 250, 000  → X BA =   120,000 + 100,000 +500,000  → Y  340, 000  → X =    720,000  → Y So the total amount spent by the group in the two cities is 340,000 paise and 720,000 paise, i.e., ` 3400 and ` 7200, respectively. EXERCISE 3.2 2 4   1 3 −2 5 1. Let A =   ,B=  ,C= 3 2  −2 5 3 4  Find each of the following: (i) A + B (ii) A – B (iii) 3A – C (iv) AB (v) BA 2. Compute the following: a b  a b a2 + b2 b2 + c2   2ab 2bc  (i)  + (ii)  2  +   −b a   b a   a + c 2 a 2 + b 2  −2ac −2ab  −1 4 −6  12 7 6   cos 2 x sin 2 x   sin 2 x cos 2 x  5 16  +  8 0 5  (iv)  (iii)  8 + 2  2 2 2  2 8 5   3 2 4   sin x cos x  cos x sin x  3. Compute the indicated products. 1  a b   a −b   2  1 −2  1 2 3 (i)  (ii)   [2 3 4] a   b a  (iii)    −b  3  2 3   2 3 1  2 3 4  1 −3 5 2 1 3  1 0 1 (iv)  3 4 5  0 2 4 (v)  2      −1 2 1  4 5 6  3 0 5  −1 1  2 −3  3 −1 3    (vi)   1 0  −1 0 2  3 1   Reprint 2024-25  MATRICES 59 1 2 −3  3 −1 2   4 1 2       4. If A = 5 0 2  , B =  4 2 5  and C =  0 3 2 , then compute 1 −1 1   2 0 3  1 −2 3 (A+B) and (B – C). Also, verify that A + (B – C) = (A + B) – C. 2 5 2 3  3 1  5 1 3 5     1 2 4 1 2 4 5. If A =   and B =   , then compute 3A – 5B. 3 3 3 5 5 5      7 2 7 6 2 2  3 3   5 5 5   cos θ sin θ   sin θ − cos θ  6. Simplify cosθ   + sinθ   − sin θ cos θ  cos θ sin θ 7. Find X and Y, if 7 0 3 0 (i) X + Y =   and X – Y =    2 5 0 3  2 3  2 −2 (ii) 2X + 3Y =   and 3X + 2Y =   4 0   −1 5  3 2   1 0 8. Find X, if Y =   and 2X + Y =  −3 2  1 4    1 3   y 0  5 6  9. Find x and y, if 2  + =   0 x   1 2  1 8  x z  1 −1   3 5 10. Solve the equation for x, y, z and t, if 2   +3  =3  y t 0 2   4 6 2  −1  10  11. If x   + y   =   , find the values of x and y. 3   1  5  x y  x 6   4 x + y 12. Given 3   =  + , find the values of x, y, z and w.  z w  −1 2 w  z + w 3  Reprint 2024-25 60 MATHEMATICS cos x − sin x 0  13. If F ( x ) =  sin x cos x 0  , show that F(x) F(y) = F(x + y).    0 0 1  14. Show that  5 −1  2 1   2 1   5 −1 (i)   ≠   6 7  3 4  3 4  6 7  1 2 3 −1 1 0   −1 1 0  1 2 3    1  ≠  0 −1 1  0 1 0  (ii) 0 1 0   0 −1 1 1 0   2 3 4   2 3 4 1 1 0  2 0 1 15. Find A – 5A + 6I, if A =  2 1 2 3  1 −1 0  1 0 2  16. If A = 0 2 1  , prove that A3 – 6A2 + 7A + 2I = 0   2 0 3  3 −2  1 0  17. If A =   and I=  2  , find k so that A = kA – 2I  4 −2  0 1   α  0 − tan  18. If A =  2 and I is the identity matrix of order 2, show that   tan α 0   2   cos α − sin α  I + A = (I – A)   sin α cos α  19. A trust fund has ` 30,000 that must be invested in two different types of bonds. The first bond pays 5% interest per year, and the second bond pays 7% interest per year. Using matrix multiplication, determine how to divide ` 30,000 among the two types of bonds. If the trust fund must obtain an annual total interest of: (a) ` 1800 (b) ` 2000 Reprint 2024-25  MATRICES 61 20. The bookshop of a particular school has 10 dozen chemistry books, 8 dozen physics books, 10 dozen economics books. Their selling prices are ` 80, ` 60 and ` 40 each respectively. Find the total amount the bookshop will receive from selling all the books using matrix algebra. Assume X, Y, Z, W and P are matrices of order 2 × n, 3 × k, 2 × p, n × 3 and p × k, respectively. Choose the correct answer in Exercises 21 and 22. 21. The restriction on n, k and p so that PY + WY will be defined are: (A) k = 3, p = n (B) k is arbitrary, p = 2 (C) p is arbitrary, k = 3 (D) k = 2, p = 3 22. If n = p, then the order of the matrix 7X – 5Z is: (A) p × 2 (B) 2 × n (C) n × 3 (D) p × n 3.5. Transpose of a Matrix In this section, we shall learn about transpose of a matrix and special types of matrices such as symmetric and skew symmetric matrices. Definition 3 If A = [aij] be an m × n matrix, then the matrix obtained by interchanging the rows and columns of A is called the transpose of A. Transpose of the matrix A is denoted by A′ or (AT). In other words, if A = [aij]m × n, then A′ = [aji]n × m. For example,  3 5 3 3 0     if A =  3 1  , then A′ = 5 1 −1   0 −1   5  2 × 3    5 3 × 2 3.5.1 Properties of transpose of the matrices We now state the following properties of transpose of matrices without proof. These may be verified by taking suitable examples. For any matrices A and B of suitable orders, we have (i) (A′)′ = A, (ii) (kA)′ = kA′ (where k is any constant) (iii) (A + B)′ = A′ + B′ (iv) (A B)′ = B′ A′ 3 3 2  2 −1 2  Example 20 If A =   and B =  2 4  , verify that 4 2 0 1 (i) (A′)′ = A, (ii) (A + B)′ = A′ + B′, (iii) (kB)′ = kB′, where k is any constant. Reprint 2024-25 62 MATHEMATICS Solution (i) We have  3 4 3 3 2   ′ 3 3 2 A=   ⇒ A′ =  3 2 ⇒ ( A′ ) =  =A  4 2 0   2 0  4 2 0    Thus (A′)′ = A (ii) We have 3 3 2  2 −1 2  5 3 − 1 4 A=  , B = 1  ⇒A+B=  4 2 0  2 4 5 4 4  5 5   Therefore (A + B)′ =  3 − 1 4  4 4   3 4  2 1     Now ′ A′ =  3 2 , B = −1 2 ,  2 0    2 4  5 5   So A′ + B′ =  3 −1 4  4 4  Thus (A + B)′ = A′ + B′ (iii) We have  2 −1 2   2 k −k 2k  kB = k  = 1 2 4   k 2k 4 k   2k k   2 1  − k 2k  = k −1 2  = kB′ Then (kB)′ =      2k 4k   2 4  Thus (kB)′ = kB′ Reprint 2024-25  MATRICES 63  −2   Example 21 If A =  4  , B = [1 3 −6] , verify that (AB)′ = B′A′.  5  Solution We have  −2   4  , B = 1 3 −6 A=   [ ]  5   −2   −2 −6 12  then  4  1 3 −6 AB =   [ ] =  4 12 −24   5   5 15 −30   1 Now A′ = [–2 4 5] , B′ =  3    − 6   1  −2 4 5  3  −2 4 5 =  −6 12 15  = (AB)′ B′A′ =   [ ]  − 6  12 −24 −30 Clearly (AB)′ = B′A′ 3.6 Symmetric and Skew Symmetric Matrices Definition 4 A square matrix A = [aij] is said to be symmetric if A′ = A, that is, [aij] = [aji] for all possible values of i and j.  3 2 3   For example A =  2 −1.5 −1  is a symmetric matrix as A′ = A  3 −1 1   Definition 5 A square matrix A = [aij] is said to be skew symmetric matrix if A′ = – A, that is aji = – aij for all possible values of i and j. Now, if we put i = j, we have aii = – aii. Therefore 2aii = 0 or aii = 0 for all i’s. This means that all the diagonal elements of a skew symmetric matrix are zero. Reprint 2024-25 64 MATHEMATICS  0 e f For example, the matrix B =  −e 0 g  is a skew symmetric matrix as B′= –B  − f −g 0  Now, we are going to prove some results of symmetric and skew-symmetric matrices. Theorem 1 For any square matrix A with real number entries, A + A′ is a symmetric matrix and A – A′ is a skew symmetric matrix. Proof Let B = A + A′, then B′ = (A + A′)′ = A′ + (A′)′ (as (A + B)′ = A′ + B′) = A′ + A (as (A′)′ = A) = A + A′ (as A + B = B + A) = B Therefore B = A + A′ is a symmetric matrix Now let C = A – A′ C′ = (A – A′)′ = A′ – (A′)′ (Why?) = A′ – A (Why?) = – (A – A′) = – C Therefore C = A – A′ is a skew symmetric matrix. Theorem 2 Any square matrix can be expressed as the sum of a symmetric and a skew symmetric matrix. Proof Let A be a square matrix, then we can write 1 1 A = (A + A′) + (A − A′) 2 2 From the Theorem 1, we know that (A + A′) is a symmetric matrix and (A – A′) is 1 a skew symmetric matrix. Since for any matrix A, (kA)′ = kA′, it follows that (A + A′) 2 Reprint 2024-25  MATRICES 65 1 is symmetric matrix and (A − A′) is skew symmetric matrix. Thus, any square 2 matrix can be expressed as the sum of a symmetric and a skew symmetric matrix.  2 −2 −4  Example 22 Express the matrix B = −1 3 4  as the sum of a symmetric and a   1 −2 −3 skew symmetric matrix. Solution Here  2 −1 1  B′ = − 2 3 −2    −4 4 −3  −3 −3   2 2 2  4 −3 −3   1  −3 1 , 2 =  1 Let P = (B + B′) = − 3 6 3  2 2 2  −3 2 −6   −3   1 −3  −3 −3   2    2 2 2   −3 Now P′ =  3 1 = P 2     −3 1 −3   2  1 Thus P = (B + B′) is a symmetric matrix. 2  −1 −5  0 2 2 0 −1 −5   1 1  1 Also, let Q = (B – B′) = 1 0 6  =  0 3  2 2 2 5 −6 0     5 −3 0  2  Reprint 2024-25 66 MATHEMATICS  1 5 0 2 3   −1 Then Q′ =  0 −3 = − Q 2     −5 3 0  2  1 Thus Q= (B – B′) is a skew symmetric matrix. 2  −3 −3   −1 −5  2 2 2  0 2 2  2     −2 −4 −3 3  = −1 3 4  = B 1 Now P+Q= 3 1 + 0 2  2       1 −2 −3  −3 1 −3   5 −3 0   2   2  Thus, B is represented as the sum of a symmetric and a skew symmetric matrix. EXERCISE 3.3 1. Find the transpose of each of the following matrices: 5 1  −1 5 6   1 −1    (i)   (ii)   (iii)  3 5 6  2  2 3 −1   2 3 −1      −1 2 3  −4 1 −5 2. If A = 5 7 9 and B =  1 2 0  , then verify that        −2 1 1   1 3 1 (i) (A + B)′ = A′ + B′, (ii) (A – B)′ = A′ – B′  3 4  −1 2 1 3. If A′ = −1 2  and B =  , then verify that    1 2 3  0 1  (i) (A + B)′ = A′ + B′ (ii) (A – B)′ = A′ – B′ Reprint 2024-25  MATRICES 67  −2 3  −1 0  4. If A′ =   and B =   , then find (A + 2B)′  1 2  1 2 5. For the matrices A and B, verify that (AB)′ = B′A′, where 1 0      (i) A = −4  , B = [ −1 2 1] (ii) A = 1  , B = [1 5 7 ]  3  2   cos α sin α  6. If (i) A =   , then verify that A′ A = I  − sin α cos α   sin α cos α  (ii) If A =   , then verify that A′ A = I  − cos α sin α  1 −1 5 7. (i) Show that the matrix A = −1 2 1 is a symmetric matrix.   5 1 3  0 1 −1  (ii) Show that the matrix A = −1 0 1  is a skew symmetric matrix.    1 −1 0  1 5  8. For the matrix A =   , verify that 6 7  (i) (A + A′) is a symmetric matrix (ii) (A – A′) is a skew symmetric matrix  0 a b 1 1 9. Find ( ′ ) A + A and ( A − A , when A = − a 0 ′ ) c  2 2   −b −c 0  10. Express the following matrices as the sum of a symmetric and a skew symmetric matrix: Reprint 2024-25 68 MATHEMATICS  6 −2 2 3 5   (i)   (ii) −2 3 −1  1 −1  2 −1 3   3 3 −1 −2 −2 1  1 5 (iii)   (iv)    −1 2   −4 −5 2 Choose the correct answer in the Exercises 11 and 12. 11. If A, B are symmetric matrices of same order, then AB – BA is a (A) Skew symmetric matrix (B) Symmetric matrix (C) Zero matrix (D) Identity matrix  cos α − sin α  12. If A =  , and A + A′ = I, then the value of α is sin α cos α  π π (A) (B) 6 3 3π (C) π (D) 2 3.7 Invertible Matrices Definition 6 If A is a square matrix of order m, and if there exists another square matrix B of the same order m, such that AB = BA = I, then B is called the inverse matrix of A and it is denoted by A– 1. In that case A is said to be invertible. 2 3  2 −3  For example, let A=   and B =  −1 2  be two matrices. 1 2     2 3   2 −3  Now AB =     1 2   −1 2   4 − 3 −6 + 6  1 0 =  = =I  2 − 2 −3 + 4   0 1  1 0 Also BA =   = I. Thus B is the inverse of A, in other 0 1  words B = A– 1 and A is inverse of B, i.e., A = B–1 Reprint 2024-25  MATRICES 69 A Note 1. A rectangular matrix does not possess inverse matrix, since for products BA and AB to be defined and to be equal, it is necessary that matrices A and B should be square matrices of the same order. 2. If B is the inverse of A, then A is also the inverse of B. Theorem 3 (Uniqueness of inverse) Inverse of a square matrix, if it exists, is unique. Proof Let A = [aij] be a square matrix of order m. If possible, let B and C be two inverses of A. We shall show that B = C. Since B is the inverse of A AB = BA = I... (1) Since C is also the inverse of A AC = CA = I... (2) Thus B = BI = B (AC) = (BA) C = IC = C Theorem 4 If A and B are invertible matrices of the same order, then (AB)–1 = B–1 A–1. Proof From the definition of inverse of a matrix, we have (AB) (AB)–1 = 1 or A–1 (AB) (AB)–1 = A –1 I (Pre multiplying both sides by A–1) or (A–1A) B (AB)–1 = A –1 (Since A–1 I = A–1) or IB (AB)–1 = A –1 or B (AB)–1 = A –1 or B–1 B (AB)–1 = B –1 A–1 or I (AB)–1 = B –1 A–1 Hence (AB)–1 = B –1 A–1 EXERCISE 3.4 1. Matrices A and B will be inverse of each other only if (A) AB = BA (B) AB = BA = 0 (C) AB = 0, BA = I (D) AB = BA = I Reprint 2024-25 70 MATHEMATICS Miscellaneous Examples  cos θ sin θ   cos nθ sin nθ  Example 23 If A =   , then prove that A n =   , n ∈ N.  − sin θ cos θ  − sin nθ cos nθ Solution We shall prove the result by using principle of mathematical induction.  cos θ sin θ   cos nθ sin nθ  We have P(n) : If A =   , then A n =  ,n∈N  − sin θ cos θ  − sin nθ cos nθ  cos θ sin θ   cos θ sin θ  , so A =  1 P(1) : A =     − sin θ cos θ  − sin θ cos θ  Therefore, the result is true for n = 1. Let the result be true for n = k. So  cos θ sin θ   cos k θ sin k θ  , then A =  k P(k) : A =     − sin θ cos θ  − sin k θ cos k θ  Now, we prove that the result holds for n = k +1  cos θ sin θ   cos k θ sin k θ  Ak + 1 = A ⋅ A =  k Now    − sin θ cos θ   − sin k θ cos k θ   cos θ cos k θ – sin θ sin k θ cos θ sin k θ + sin θ cos k θ  =    − sin θ cos k θ + cos θ sin k θ − sin θ sin k θ + cos θ cos k θ   cos (θ + k θ) sin (θ + k θ)   cos( k + 1)θ sin (k + 1)θ  =  =   − sin (θ + k θ) cos (θ + k θ)   − sin (k + 1)θ cos (k + 1)θ  Therefore, the result is true for n = k + 1. Thus by principle of mathematical induction,  cos n θ sin n θ  we have A =  n , holds for all natural numbers. − sin n θ cos n θ  Example 24 If A and B are symmetric matrices of the same order, then show that AB is symmetric if and only if A and B commute, that is AB = BA. Solution Since A and B are both symmetric matrices, therefore A′ = A and B′ = B. Reprint 2024-25  MATRICES 71 Let AB be symmetric, then (AB)′ = AB But (AB)′ = B′A′= BA (Why?) Therefore BA = AB Conversely, if AB = BA, then we shall show that AB is symmetric. Now (AB)′ = B′A′ = B A (as A and B are symmetric) = AB Hence AB is symmetric.  2 −1 5 2  2 5 Example 25 Let A =   ,B=  ,C= . Find a matrix D such that  3 4 7 4  3 8 CD – AB = O. Solution Since A, B, C are all square matrices of order 2, and CD – AB is well defined, D must be a square matrix of order 2. a b  Let D=  . Then CD – AB = 0 gives c d   2 5  a b   2 −1  5 2  3 8  c d  − 3 4 7 4 = O        2a + 5c 2b + 5d   3 0   0 0  or  3a + 8c 3b + 8d  −  43 22 =       0 0   2a + 5c − 3 2b + 5d   0 0  or  3a + 8c − 43 3b + 8d − 22 =  0 0      By equality of matrices, we get 2a + 5c – 3 = 0... (1) 3a + 8c – 43 = 0... (2) 2b + 5d = 0... (3) and 3b + 8d – 22 = 0... (4) Solving (1) and (2), we get a = –191, c = 77. Solving (3) and (4), we get b = – 110, d = 44. Reprint 2024-25 72 MATHEMATICS  a b   −191 −110  Therefore D=  = 44   c d   77 Miscellaneous Exercise on Chapter 3 1. If A and B are symmetric matrices, prove that AB – BA is a skew symmetric matrix. 2. Show that the matrix B′AB is symmetric or skew symmetric according as A is symmetric or skew symmetric. 0 2 y z 3. Find the values of x, y, z if the matrix A =  x y − z  satisfy the equation   x − y z  A′A = I. 1 2 0   0  4. For what values of x : [1 2 1]  2 0 1   2  = O? 1 0 2   x  3 1 5. If A =  , show that A2 – 5A + 7I = 0. −1 2  1 0 2  x  6. Find x, if [ x −5 −1]  0 2 1   4  = O     2 0 3 1  7. A manufacturer produces three products x, y, z which he sells in two markets. Annual sales are indicated below: Market Products I 10,000 2,000 18,000 II 6,000 20,000 8,000 Reprint 2024-25  MATRICES 73 (a) If unit sale prices of x, y and z are ` 2.50, ` 1.50 and ` 1.00, respectively, find the total revenue in each market with the help of matrix algebra. (b) If the unit costs of the above three commodities are ` 2.00, ` 1.00 and 50 paise respectively. Find the gross profit. 1 2 3  −7 −8 −9  8. Find the matrix X so that X  = 4 6  4 5 6  2 Choose the correct answer in the following questions: α β  9. If A =  is such that A² = I, then γ −α  (A) 1 + α² + βγ = 0 (B) 1 – α² + βγ = 0 (C) 1 – α² – βγ = 0 (D) 1 + α² – βγ = 0 10. If the matrix A is both symmetric and skew symmetric, then (A) A is a diagonal matrix (B) A is a zero matrix (C) A is a square matrix (D) None of these 2 11. If A is square matrix such that A = A, then (I + A)³ – 7 A is equal to (A) A (B) I – A (C) I (D) 3A Summary ® A matrix is an ordered rectangular array of numbers or functions. ® A matrix having m rows and n columns is called a matrix of order m × n. ® [aij]m × 1 is a column matrix. ® [aij]1 × n is a row matrix. ® An m × n matrix is a square matrix if m = n. ® A = [aij]m × m is a diagonal matrix if aij = 0, when i ≠ j. ® A = [aij]n × n is a scalar matrix if aij = 0, when i ≠ j, aij = k, (k is some constant), when i = j. ® A = [aij]n × n is an identity matrix, if aij = 1, when i = j, aij = 0, when i ≠ j. ® A zero matrix has all its elements as zero. ® A = [aij] = [bij] = B if (i) A and B are of same order, (ii) aij = bij for all possible values of i and j. Reprint 2024-25 74 MATHEMATICS ® kA = k[aij]m × n = [k(aij)]m × n ® – A = (–1)A ® A – B = A + (–1) B ® A+ B = B +A ® (A + B) + C = A + (B + C), where A, B and C are of same order. ® k(A + B) = kA + kB, where A and B are of same order, k is constant. ® (k + l ) A = kA + lA, where k and l are constant. n ® c If A = [aij]m × n and B = [bjk]n × p , then AB = C = [cik]m × p, where ik = ∑a b j =1 ij jk ® (i) A(BC) = (AB)C, (ii) A(B + C) = AB + AC, (iii) (A + B)C = AC + BC ® If A = [aij]m × n, then A′ or AT = [aji]n × m ® (i) (A′)′ = A, (ii) (kA)′ = kA′, (iii) (A + B)′ = A′ + B′, (iv) (AB)′ = B′A′ ® A is a symmetric matrix if A′ = A. ® A is a skew symmetric matrix if A′ = – A. ® Any square matrix can be represented as the sum of a symmetric and a skew symmetric matrix. ® If A and B are two square matrices such that AB = BA = I, then B is the inverse matrix of A and is denoted by A–1 and A is the inverse of B. ® Inverse of a square matrix, if it exists, is unique. —v — Reprint 2024-25  MATRICES 75 NOTES Reprint 2024-25