discrete mathematics


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Question 10
Question: From 12 books, in how many ways can 5 be chosen i. to surely include a particular book ii. never to include a particular book iii. to have no restrictions at all? Answer: i. To include a particular book, one particular book has already been chosen. We have to select 4 books fr..
Adjoint of a Square Matrix
The adjoint of a square matrix [a i j ] is defined as the transpose of the matrix [A i j ] where A i j are the cofactors of the elements a i j . Adjoint of A is denoted by adj A. ..
Inverse of a Square Matrix
Let A be a square matrix of order n. If there exists a matrix B of order n such that AB = BA = I, where I is the identity matrix of order n, then the matrix A is said to be invertible and B is called the inverse (or reciprocal) of ..
Note 2:
From the definition, it is clear that if B is the inverse of A, then A is the inverse of ..
Theorem:
The inverse of a square matrix if it exists, is unique. Let A be an invertible square matrix. If possible, let B and C be two inverse of A. Then AB = BA = I. AC = CA = I (by def. of inverse) Now, B = BI = B(AC) = (BA)C [ Matrix multiplication is associative] = IC = C i.e., B = C Hence the inverse..
Proof:
From the definition of inverse of a matrix, we have (AB)(AB) - 1 = I or A - 1 (AB)(AB) - 1 = A - 1 I (Pre-multiplying both sides by A - 1 ) or (A - 1 A) B (AB) - 1 = A - 1 (Since A - 1 I = A - 1 ) or I B (AB) - 1 = A - 1 or B (AB) - 1 = A - 1 or (B - 1 B)(AB) - 1 =B - ..
Elementary Transformation
Elementary transformations are of the following three types: Interchange of any two rows (or columns) The multiplication of the elements of a row (or column) by a non-zero number. The addition to the elements of any row (or column) the corresponding elements of any other row (or column) mu..
Properties of adjoint of a matrix
1. A.(adj A) = (adj A). A = |A| I 2. adj (AB) = (adj B) . (adj ..
Singular Matrix
A square matrix A is said to be singular if |A| = ..
Consistency and Inconsistency of a System of Linear Equations
A system of linear equations is said to be consistent if it has a solution. This means that the solution satisfies all the equations in the system simultaneously. If a system of linear equations has no solution, then it is said to be inconsiste..
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