**12 class Maths Notes Chapter 1 Relations and Functions free PDF| Quick revision Notes class 12 maths**

**CBSE Revision Notes for CBSE Class 12 Mathematics Relations and Functions**Types of relations: reflexive, symmetric, transitive and equivalence relations. One to one and onto functions, composite functions, inverse of a function. Binary operations.

**Class 12 Maths Chapter-1 Relations and Functions Quick Revision Notes Free Pdf**

**✳️ Definitions**

๐นLet A and B be two non-empty sets, then a function f from set A to set B is a rule which associates each element of A to a unique element of B.

**✳️ Relation**

๐น If (a, b) € R, we say that a is related to b under the relation R and we write as a R b

**✳️ Function**

๐น It is represented as f: A → B and function is also called mapping .

**✳️ Real Function**

๐น f: A → B is called a real function, if A and B are subsets of R.

**✳️ Domain and Codomain of a Real Function**

๐น Domain and codomain of a function f is a set of all real numbers x for which f(x) is a real number. Here, set A is domain and set B is codomain .

**✳️ Range of a real function**

๐น f is a set of values f(x) which it attains on the points of its domain .

**✳️ Types of Relations :-**

๐น A relation R in a set A is called

**Empty relation**, if no element of A is related to any element of

A, i.e., R = ฯ ∈ A x A.

๐น A relation R in a set A is called

**Universal relation**, if each element of A is related to every element of

A, i.e., R = A x A.

๐น Both the empty relation and the universal relation are sometimes called

**Trivial Relations**.

๐น A relation R in a set A is called .

๐

**Reflexive**

๐น if (a, a) ∈ R, for every a ∈ A,

๐

**Symmetric**

๐น If (a1, a2) ∈ R implies that (a2, a1) ∈ R, for all a1, a2∈ A.

๐

**Transitive**

๐น If (a1, a2) ∈ R and (a2, a3) ∈ R implies that (a1, a3) ∈ R, for all a1, a2, a3 ∈ A.

๐น A relation R in a set A is said to be an

**equivalence relation**if R is reflexive, symmetric and transitive .

๐น The set E of all even integers and the set O of all odd integers are subsets of Z satisfying following conditions:

๐ All elements of E are related to each other and all elements of O are related to each other.

๐ No element of E is related to any element of O and vice-versa.

๐ E and O are disjoint and Z = E U O.

๐ The subset E is called the equivalence class containing zero, Denoted by [0].

Ois the equivalence class containing 1 and is denoted by [1].

๐ท

**Note :-**

๐น [o] ≠ [1]

๐น [o] = [2r]

๐น [1] = [2r + 1], r ∈ Z.

๐ท Given an arbitrary equivalence relation R in an arbitrary set X, R divides X into mutually disjoint subsets Ai called partitions or subdivisions of X satisfying:

๐น All elements of Ai are related to each other, for all i.

๐น No element of Ai is related to any element of Aj, i ≠ j.

๐น UAj = X and Ai∩Aj = ฯ, i + j.

๐ท The subsets Ai are called equivalence classes.

✳️

**Note**:

๐ท Two ways of representing a relation

๐น Roaster method

๐น Set builder method

๐ท If (a, b) ∈ R, we say that a is related to b and we denote it as

**a R b**.

**✳️ Types of Funetions**

๐น Consider the functions f1, f2, f3 and f4 given

๐น A function f: X → Y is defined to be

**one-one (or injective)**, if the images of distinct elements of X under f are distinct, i.e., for every X1, X2 ∈ X, f(X1) = f(X2) implies X1, X2. Otherwise, f is called

**many - one .**

**๐น**A function f: X→Y is said to be

**onto (or surjective)**, if every element of Y is the image of some element of X under f, i.e., for every y ∈ Y, there exists an element x in X such that f(x) y.

๐น f: X→Y is onto if and only if Range of f = Y.

๐น Eg :

๐น A function f: X→Y is said to be

**one-one and onto (or bijective)**, if f is both one-one and onto.

๐น Eg:

**✳️ Composition of Functions and Invertible Function**

**✳️ Composite Function**

๐น Let f: A→B and g: B→C be two functions.

๐น Then the composition of f and g, denoted by gof, is defined as the function gof: A→C given by

**gof (x) = g(f (x)), ∀ x ∈ A.**

๐น

**Eg**:

● Let f : {2, 3, 4, 5} → {3, 4, 5, 9} and g : {3, 4, 5, 9} → {7, 11, 15} be functions

● Defined as f (2) = 3, f(3) = 4, f (4) = f (5) = 5 and g (3) = g (4) = 7 and g (5) = g (9) = 11.

● Find gof.

๐น

**Solution**

● gof(2) = g (f(2)) = g (3) = 7,

● gof (3) = g (f(3)) = g (4) = 7,

● gof (4) = g (f(4)) = g (5) = 11 and

● gof (5) = g (5) = 11

๐น It can be verified in general that gof is one-one implies that f is one-one. Similarly, gof is onto implies that g is onto.

๐น While composing f and g, to get gof, first f and then g was applied, while in the reverse process of the composite gof, first the reverse process of g is applied and then the reverse process of f.

๐น If f: X → Y is a function such that there exists a function g: Y → X such that gof = IX and fog = IY, then f must be one-one and onto.

**✳️ Invertible Function**

๐น A function f: X → Y is defined to be invertible, if there exists a function g: Y→X such that gof = IX and fog = IY. The function g is called the inverse of f

๐น Denoted by f ⁻¹.

๐น Thus, if f is invertible, then f must be one-one and onto and conversely, if f is one-one and onto, then f must be invertible.

**✳️ Theorem 1**

๐น If f : X→Y, g : Y→Z and h : Z→S are functions, then

• h₀ (g₀f) = (h₀g) ₀f.

๐น Proof

We have

• h₀ (g₀f) (x) = h(g₀f (x)) = h(g(f(x))), ∀ x in X

• (h₀g)₀f (x) = h₀g (f(x)) = h (g (f(x))), ∀ x in X.

Hence, h₀ (g₀f) = (h₀g) ₀ f

**✳️ Theorem 2**

๐นLet f: X→Y and g: Y→ Z be two invertible functions.

• Then g₀f is also invertible with (g₀f)⁻¹ =f⁻¹₀ g⁻¹

๐น Proof

• To show that g₀f is invertible with (g ₀ f)⁻¹ =f⁻¹₀g⁻¹, it is enough to show that (f⁻¹₀g⁻¹) o (g ₀ f) = Iโ and (g ₀ f) ₀ (f⁻¹ ₀ g⁻¹) = Iz.

Now, (f⁻¹₀g⁻¹) o (g ₀ f) = ((F⁻¹ ₀ g⁻¹) ₀ g) of, by Theorem 1

= (f⁻¹₀ (g⁻¹₀g)) of, by Theorem 1

= (f⁻¹ ₀ Iy) of, by definition of g⁻¹

= Iโ

Similarly, it can be shown that (g ₀ f) ₀ (f⁻¹ ₀ g⁻¹) = Iz

**✳️ Binary Operations**

**✳️ Definitions:**

๐น A binary operation * on a set A is a function * : A × A → A. We denote * (a, b) by a * b.

๐นA binary operation * on the set X is called commutative, if a * b = b * a, for every a, b ∈ X

๐น A binary operation * : A × A→A is said to be associative if (a * b) * c = a * (b * c), ∀ a, b, c, ∈ A.

๐น A binary operation * : A × A→A, an element e ∈ A, if it exists, is called identity for the operation *, if a * e = a = e * a, ∀ a ∈ A.

• Zero is identity for the addition operation on R but it is not identity for the addition operation on N, as o ∈ N.

• Addition operation on N does not have any identity.

• For the addition operation + : R × R→R, given any a ∈ R, there exists - a in R such that a + (-a) = 0 (identity for '+') = (-a) + a.

๐น A binary operation * : A × A + A with the identity element e in A, an element a e A is said to be invertible with respect to the operation •, if there exists an element b in A such that a + b = e = b+a and b is called the inverse of a and is denoted by a-

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