Plus one, Mathematics, Chapter 1.
Sets, problems solved.
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Introduction.
Different types of sets.
Chapter 1
SETS
1.1 Introduction
1.2 Sets and their Representations
In everyday life, we often speak of collections of objects
We give below a few more examples of sets
N : the set of all natural numbers
Z : the set of all integers
Q : the set of all rational numbers
R : the set of real numbers
Z+ : the set of positive integers
Q+ : the set of positive rational numbers, and
R+ : the set of positive real numbers.
We shall say that a set is a well-defined collection of objects.
There are two methods of representing a set :
(i) Roster or tabular form
(ii) Set-builder form.
1.5 Equal Sets
1.6 Subsets
Consider the sets : X = set of all students in your school, Y = set of all students in your
class.
A ⊂ B if a ∈ A ⇒ a ∈ B
We read the above statement as “A is a subset of B if a is an element of A
implies that a is also an element of B”. If A is not a subset of B, we write A ⊄ B.
The set of irrational numbers, denoted by T, is composed of all other real numbers.
Thus T = {x : x ∈ R and x ∉ Q},
N ⊂ Z ⊂ Q, Q ⊂ R, T ⊂ R, N ⊄ T.
1.7 Power Set
Definition 5 The collection of all subsets of a set A is called the power set of A. It is
denoted by P(A). In P(A), every element is a set.
1.8 Universal Set
1.10 Operations on Sets
1.10.1 Union of sets Let A and B be any two sets.
A ∪ B = { x : x ∈A or x ∈B }
Some Properties of the Operation of Union
(i) A ∪ B = B ∪ A (Commutative law)
(ii) ( A ∪ B ) ∪ C = A ∪ ( B ∪ C)
(Associative law )
(iii) A ∪ φ = A (Law of identity element, φ is the identity of ∪)
(iv) A ∪ A = A (Idempotent law)
(v) U ∪ A = U (Law of U)
1.10.2 Intersection of sets
A ∩ B = {x : x ∈ A and x ∈ B}.
The intersection of two sets A and B
A ∩ B = {x : x ∈ A and x ∈ B}
If A and B are two sets such that A ∩ B = φ, then
A and B are called disjoint sets.
Some Properties of Operation of Intersection
(i) A ∩ B = B ∩ A (Commutative law).
(ii) ( A ∩ B ) ∩ C = A ∩ ( B ∩ C ) (Associative law).
(iii) φ ∩ A = φ, U ∩ A = A (Law of φ and U).
(iv) A ∩ A = A (Idempotent law)
(v) A ∩ ( B ∪ C ) = ( A ∩ B ) ∪ ( A ∩ C ) (Distributive law ) i. e.,
1.10.3 Difference of sets
A – B = { x : x ∈ A and x ∉ B }
1.11 Complement of a Set
Some Properties of Complement Sets
1. Complement laws: (i) A ∪ A′ = U (ii) A ∩ A′ = φ
2. De Morgan’s law: (i) (A ∪ B)´ = A′ ∩ B′ (ii) (A ∩ B )′ = A′ ∪ B′
3. (A′ )′ = A
4. Laws of empty set and universal set φ′ = U and U′ = φ.
(ii) n ( A ∪ B ) = n ( A ) + n ( B ) – n ( A ∩ B ) ... (2)
n ( A ∪ B) = n ( A – B) + n ( A ∩ B ) + n ( B – A )
(iii) If A, B and C are finite sets, then
n ( A ∪ B ∪ C ) = n ( A ) + n ( B ) + n ( C ) – n ( A ∩ B ) – n ( B ∩ C)
– n ( A ∩ C ) + n ( A ∩ B ∩ C )
Summary
This chapter deals with some basic definitions and operations involving sets. These
are summarised below:
A set is a well-defined collection of objects.
A set which does not contain any element is called empty set.
A set which consists of a definite number of elements is called finite set,
otherwise, the set is called infinite set.
Two sets A and B are said to be equal if they have exactly the same elements.
A set A is said to be subset of a set B, if every element of A is also an element
of B. Intervals are subsets of R.
A power set of a set A is collection of all subsets of A. It is denoted by P(A).
The union of two sets A and B is the set of all those elements which are either
in A or in B.
The intersection of two sets A and B is the set of all elements which are
common. The difference of two sets A and B in this order is the set of elements
which belong to A but not to B.
The complement of a subset A of universal set U is the set of all elements of U
which are not the elements of A.
For any two sets A and B, (A ∪ B)′ = A′ ∩ B′ and ( A ∩ B )′ = A′ ∪ B′
If A and B are finite sets such that A ∩ B = φ, then
n (A ∪ B) = n (A) + n (B).
If A ∩ B ≠ φ, then
n (A ∪ B) = n (A) + n (B) – n (A ∩ B)
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