**Set**

A set is a well-defined collection of objects.

**Representation of Sets**

There are two methods of representing a set

- Roster or Tabular form In the roster form, we list all the members of the set within braces { } and separate by commas.
- Set-builder form In the set-builder form, we list the property or properties satisfied by all the elements of the sets.

### Types of Sets – Class 11 Maths Notes

**Empty Sets:**A set which does not contain any element is called an empty set or the void set or null set and it is denoted by {} or Φ.**Singleton Set:**A set consists of a single element, is called a singleton set.**Finite and infinite Set:**A set which consists of a finite number of elements, is called a finite set, otherwise the set is called an infinite set.**Equal Sets:**Two sets A and 6 are said to be equal, if every element of A is also an element of B or vice-versa, i.e. two equal sets will have exactly the same element.**Equivalent Sets:**Two finite sets A and 6 are said to be equal if the number of elements are equal, i.e. n(A) = n(B)

### Subset – Class 11 Maths Notes

A set A is said to be a subset of set B if every element of set A belongs to set B. In symbols, we write

A ⊆ B, if x ∈ A ⇒ x ∈ B

Note:

- Every set is o subset of itself.
- The empty set is a subset of every set.
- The total number of subsets of a finite set containing n elements is 2
^{n}.

**Intervals as Subsets of R**

Let a and b be two given real numbers such that a < b, then

- an open interval denoted by (a, b) is the set of real numbers {x : a < x < b}.
- a closed interval denoted by [a, b] is the set of real numbers {x : a ≤ x ≤ b}.
- intervals closed at one end and open at the others are known as semi-open or semi-closed interval and denoted by (a, b] is the set of real numbers {x : a < x ≤ b} or [a, b) is the set of real numbers {x : a ≤ x < b}.

**Power Set**

The collection of all subsets of a set A is called the power set of A. It is denoted by P(A). If the number of elements in A i.e. n(A) = n, then the number of elements in P(A) = 2^{n}.

**Universal Set**

A set that contains all sets in a given context is called the universal set.

**Venn-Diagrams**

Venn diagrams are the diagrams, which represent the relationship between sets. In Venn-diagrams the universal set U is represented by point within a rectangle and its subsets are represented by points in closed curves (usually circles) within the rectangle.

**Operations of Sets**

Union of sets: The union of two sets A and B, denoted by A ∪ B is the set of all those elements which are either in A or in B or in both A and B. Thus, A ∪ B = {x : x ∈ A or x ∈ B}.

**Intersection of sets:** The intersection of two sets A and B, denoted by A ∩ B, is the set of all elements which are common to both A and B.

Thus, A ∩ B = {x : x ∈ A and x ∈ B}

**Disjoint sets:** Two sets Aand Bare said to be disjoint, if A ∩ B = Φ.

**Intersecting or Overlapping sets:** Two sets A and B are said to be intersecting or overlapping if A ∩ B ≠ Φ

**Difference of sets:** For any sets A and B, their difference (A – B) is defined as a set of elements, which belong to A but not to B.

Thus, A – B = {x : x ∈ A and x ∉ B}

also, B – A = {x : x ∈ B and x ∉ A}

**Complement of a set:** Let U be the universal set and A is a subset of U. Then, the complement of A is the set of all elements of U which are not the element of A.

Thus, A’ = U – A = {x : x ∈ U and x ∉ A}

**Some Properties of Complement of Sets**

- A ∪ A’ = ∪
- A ∩ A’ = Φ
- ∪’ = Φ
- Φ’ = ∪
- (A’)’ = A

**Symmetric difference of two sets:** For any set A and B, their symmetric difference (A – B) ∪ (B – A)

(A – B) ∪ (B – A) defined as set of elements which do not belong to both A and B.

It is denoted by A ∆ B.

Thus, A ∆ B = (A – B) ∪ (B – A) = {x : x ∉ A ∩ B}.

### Laws of Algebra of Sets – Class 11 Maths Notes

**Idempotent Laws:** For any set A, we have

- A ∪ A = A
- A ∩ A = A

**Identity Laws:** For any set A, we have

- A ∪ Φ = A
- A ∩ U = A

**Commutative Laws:** For any two sets A and B, we have

- A ∪ B = B ∪ A
- A ∩ B = B ∩ A

**Associative Laws:** For any three sets A, B and C, we have

- A ∪ (B ∪ C) = (A ∪ B) ∪ C
- A ∩ (B ∩ C) = (A ∩ B) ∩ C

**Distributive Laws:** If A, B and Care three sets, then

- A ∪ (B ∩ C) = (A ∪ B) ∩ (A ∪ C)
- A ∩ (B ∪ C) = (A ∩ B) ∪ (A ∩ C)

**De-Morgan’s Laws:** If A and B are two sets, then

- (A ∪ B)’ = A’ ∩ B’
- (A ∩ B)’ = A’ ∪ B’

**Formulae to Solve Practical Problems on Union and Intersection of Two Sets**

Let A, B and C be any three finite sets, then

- n(A ∪ B) = n(A) + n (B) – n(A ∩ B)
- If (A ∩ B) = Φ, then n (A ∪ B) = n(A) + n(B)
- n(A – B) = n(A) – n(A ∩ B)
- n(A ∪ B ∪ C) = n(A) + n(B) + n(C) – n(A ∩ B) – n(B ∩ C) – n(A ∩ C) + n(A ∩ B ∩ C)