# Chapter 1 - Matrices & Determinants

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1 Chapter 1 - Matrices & Determinants Arthur Cayley (August 16, January 26, 1895) was a British Mathematician and Founder of the Modern British School of Pure Mathematics. As a child, Cayley enjoyed solving complex math problems for amusement. At eighteen, he entered Trinity College, Cambridge, where he excelled in Greek, French, German, and Italian, as well as in Mathematics. He worked as a lawyer for 14 years. He was consequently able to prove the Cayley-Hamilton Theorem that every square matrix is a root of its own characteristic polynomial. He was the first to define the concept of a group in the modern way as a set with a binary operation satisfying certain laws. DETERMINANTS In algebra, a determinant is a function depending on n that associates a scalar, det(a), to every n n square matrix A. The fundamental geometric meaning of a determinant acts as the scale factor for volume when A is regarded as a linear transformation. Determinants are important both in Calculus, where they enter the substitution rule for several variables, and in Multilinear Algebra. For a fixed positive integer n, there is a unique determinant function for the n n matrices over any commutative ring R. In particular, this function exists when R is the field of real or complex numbers. The determinant of a square matrix A is also sometimes denoted by A. This notation can be ambiguous since it is also used for certain matrix norms and for the absolute value. However, often the matrix norm will be denoted with double vertical bars e.g., and may carry a subscript as well. Thus, the vertical bar notation for determinant is frequently used (e.g., Cramer s rule and minors). For example, for matrix the determinant det(a) might be indicated by A or more explicitly as 1

2 MATRICES Matrix is a rectangular array of elements in rows and columns put in a large braces defines the lexicon. But there is more than that meets the eye. The term matrix was coined in 1848 by J.J. Sylvester. Arthur Cayley, William Rowan Hamilton, Grassmann, Frobenius and von Neumann are among the famous mathematicians who have worked on matrix theory. Though it has no numerical value as a whole, it is put to use in myriad fields. Matrix represents transformations of coordinate spaces. It is a mathematical shorthand to help study problems of entries. It provides convenient and compact notation for representation of data. Out of the inexhaustive uses of matrices the following may be called as the predominant: 1. Finding sets of solutions of a system of linear equations. 2. To study the relation on sets, directed routes and cryptography i.e. coding and decoding secret messages. 3. Used in input output analysis of industries to test the viability of the economic systems of industries. 4. Used in finite element methods (Civil and Structural engineering) and in network analysis (Electrical and Electronic engineering). 5. For handling large amount of data in computers, which is widely useful at present. 6. It is widely used in other fields like Statistics, Psychology, Operation Research, Differential Equations, Mechanics, Electrical Circuits, Nuclear Physics, Aerodynamics, Astronomy, Quantum Mechanics etc., The students are already acquainted with the basic operations of Matrices such as matrix additions, matrix multiplication, etc. However, some of these important properties are now recalled to participate further learning. 1. Matrix addition is commutative and associative. ie. i) A + B = B + A ii) A + (B + C) = (A + B) + C 2. Matrix multiplication is not commutative. ie. AB BA 3. But Matrix multiplication is associative. ie. A(BC) = (AB)C 4. Distributive Property i) A(B + C) = AB + AC (Left Distributive Property of Matrix multiplication over addition) ii) (A + B)C = AC + BC (Right Distributive Property of Matrix multiplication over addition) Note : i) In fact 2

3 Important Points in Matrices & Determinants Properties of the Determinants (Without Proof) 1) The value of the Determinant is not altered by interchanging the rows and columns (It is symbolically denoted as R C). (i.e) 2) If any two rows or any two columns of a determinant are interchanged then the value of the determinant changes in sign, but its numerical value is unaltered Example (i.e) 1 = (Here C 1 & C 2 are interchanged represented, as C 1 C 2 ) Note : If a row (or a column) is made to pass through two parallel rows (or column) then the value of the determinant remains unchanged ie. rows. In general if a row (or a column) is made to pass through n such parallel (or columns) then we have the new determinant. 3) If any two rows (or two columns) are identical then the value of the determinant is zero Example : 4) If every element in a row or a column of a determinant is multiplied by a non-zero constant k (i.e k 0), then the value of the determinant is multiplied by k. Example : (i.e) 1 = k (Every element of R 1 is multiplied by k) Note : If all the three rows (or all the three columns) of a 3 rd order determinant are multiplied by k, then the value of the determinant is multiplied by 3 ;

4 (ie. 1 = k 3 ) 5) If every entry in a row (or column) can be expressed as sum of two quantities then the given determinant can be expressed as sum of two determinants of the same order i.e. Note : Two determinants of the same order can be added, by adding the corresponding entries of a particular row (or a column), provided the other two rows (or the columns) in both the determinants are the same. Factor Theorem 1) If two rows (or columns) of a determinant are identical on putting x = a, then (x a) is a factor of 2) If all the three rows (or columns) of a determinant are identical on putting x = a, then (x a) 2 is a factor of cofactor determinant. is Note: 1) = 2) 6) A determinant is unaltered when to each element of any row ( or column) the equimultiples of any corresponding row (or column) are added Example : 4 i.e. we have Note : Usual operations (Elementary transformations) that are carried out so that the value of the determinant is unaltered are [This is done if every entry in a row is the same]

5 the [This is done if every entry in a column is same] 3) in [This is done if every entry after addition first column becomes the same (or has a common factor)] 4) [This is done if every entry after addition in the first row becomes the same (or has a common factor)] Matrix and its Applications Matrices are rectangular arrangements of numbers in rows and columns put within a large paranthesis. Matrices are denoted by capital letters like A, B, C and so on. eg. Order of Matrix : Order of Matrix A is the number of rows and the number of columns that are present in a Matrix. Suppose a Matrix A has m rows and n columns the order of Matrix A is denoted by m x n read as m by n. The order of the above said Matrix A is 3 x 3. The order of the above said Matrix B is 2 x 3. Difference between Matrix and a Determinant 1. Matrices do not have definite value, but determinants have definite value. 2. In a Matrix the number of rows and columns may be unequal, but in a Determinant the number of rows and columns must be equal. 3. The entries of a Matrix are listed within a large paranthesis (large braces), but in a determinant the entries are listed between two strips (i.e. between two vertical lines). 4. Let A be a Matrix. Matrix ka is obtained by multiplying all the entries of the Matrix by k. Let be any Determinant. is obtained by multiplying every entry of a row or every entry of a column by k (i.e. in order to multiply a Determinant by a number 5 k

6 every entry of a row or every entry of a column is multiplied by k). Caution : Do not multiply all the entries of the Determinant by k in order to multiply the Determinant by k. Note : If A is a 3 rd order square matrix In general if A is an n th order square matrix 1. Adjoint of a Matrix : Let be a square matrix of order n. Let A ij be the cofactor entry of each a ij of the Matrix A. Then is the adjoint of the Matrix A. i.e Adjoint of matrix A is the transpose of co-factor matrix A Adj A = ; where A ij is the co-factor entry of matrix A Note : Adjoint of a matrix can be found for square matrices only and we have Inverse : Let A be a square matrix of order n. Then a matrix B, is called the inverse of matrix A if,.the inverse of A is denoted by The formula for finding the inverse of matrix A is denoted by Caution : If the inverse of matrix A does not exist. But the adjoint of matrix A exists. Singular Matrix : A square matrix A of order n is a singular matrix if its determinant value is zero. i.e. exist for a singular matrix matrix A is a singular matrix. Inverse does not Non-Singular Matrix : A square matrix A of order n is a non-singular matrix if its determinant value is not equal to zero. i.e. matrix A is a non-singular matrix. Inverses do exist for non-singular matrices. i.e. If A is a non-singular square matrix then B is called the inverse of A, if 6

7 Note : Inverse for a matrix exists only for a square matrix, provided its determinant value is not equal to zero. Rank of a Matrix : Matrix A is said to be of rank r, if i) A has atleast one minor of order r which does not vanish. 7 ii) Every minor of A of order (r + 1) and higher order vanishes. In other words Rank of Matrix A is equal to the order of the highest non-vanishing minor of the matrix. Note : Rank of a Matrix is less than or equal to the least of its row or its column. Echelon Form i.e. 1. If order of matrix A is 3 x 3 2. If order of matrix A is 5 x 4 3. If order of matrix A is 2 x 3 Finding the rank of a matrix involves more computation work. Reducing it into the Echelon form may be useful in finding rank. Echelon form of a matrix : A matrix A (of order m x n) is said to be in echelon form (triangular form) if i) Every row of A which has all its entries 0 occurs below every row which has a non-zero entry. ii) The first non-zero entry in each non-zero row is 1. iii) The number of zeros before the first non-zero element in a row is less than the number of such zeros in the next row. By elementary operations one can easily bring the given matrix to its echelon form.

8 Equivalent matrices : Two matrices A and B of the same order are said to be equivalent if one can be obtained from the other by the applications of a finite number of sequences of elementary transformation. Matrix A is equivalent to matrix B is symbolically denoted by A ~ B Note : Equivalent matrices have the same rank. Solution of Non homogeneous equations : Let us define, x and z as Consistent / Inconsistent 1. The given system of equations is said to be consistent if the system of linear equations possesses atleast one solution. The given system of equations is said to be inconsistent if the system of linear equations has no solution. 2. i) If the system is consistent with a unique solution and the solution may be 8

9 obtained using formula ii) If = 0 the system may be consistent or inconsistent The following table gives a clear idea of the system of linear equations being consistent or inconsistent if is 0 Consistent Inconsistent no 1. also x = 0 but it atleast one of x or z and atleast one of the is non-zero the system has 2 x 2 minors of is non-zero solution the system has infinite solutions; put z = k and solve any two equations 2. also x = 0; all 2 x 2 also x = 0 minors of, x and z are zero. all 2 x 2 minors of are zero but If has atleast one non-zero atleast one of the 2 x 2 minors of element, the system has x or z is non-zero infinite solutions. Put y = s, z = t the system has no solution and solve any one equation. The description displayed in the above table is neatly represented in the following diagrams for verifying the system of equations to be consistent or inconsistent. The system is consistent with a unique solution. In other words the three planes have only one common point. i.e. S is the only common point as shown in the figure. 9 = 0 and x = 0 and if one of the 2 x 2 minors of is not equal to zero The system is consistent with infinite no. of solutions. In this case the three planes intersect along a line as shown

10 in the figure. = 0 and x = 0 and all the 2 x 2 minors of, x, z are also zero The system is consistent with infinite no. of solutions. i.e. all the three equations represent one and the same plane. The three planes are coincident. = 0 and if one of the values of x, z is not equal to zero The system is inconsistent and has no solution. The three planes do not intersect at a common point. i.e. The three planes intersect as shown in the diagram Consistency or Inconsistency using rank = 0 and x = 0 and all 2 x 2 the minors of are also zero and if one of the 2 x 2 minors of x, z is not equal to zero The system is inconsistent and has no solution. i.e. The three planes are parallel planes. The Augmented matrix [A, B] = i) Write down the given system of equations in the form of a matrix equation AX = B 10

11 ii) Find the augmented matrix [A, B] of the system of equations. iii) Find the rank of A and rank of [A, B] by applying only elementary row operations. Caution : Elementary Column operations should not be applied in augmented matrix iv) a) If the rank of A rank of [A, B] then the system is inconsistent and has no solution. b) If the rank of A = rank of [A, B] = n, where n is the number of unknowns in the system, then A is a non-singular matrix and the system is consistent and it has a unique solution. c) If the rank of A = rank of [A, B] < n, then also the system is consistent, but has an infinite number of solutions. Homogeneous Linear System This system has a trivial solution. i.e. x = 0, y = 0, z = 0 is the trivial solution If this system has any other solutions in addition to the trivial solutions then they are known as non-trivial solutions. In the case of Homogeneous Linear System of equations we have only two possibilities i) The system has only a trivial solution ii) The system has infinitely many solutions in addition to the trivial solution, known as non-trivial solutions. 11

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