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# Class 12 NCERT Solutions – Mathematics Part I – Chapter 1 Relations and Functions – Exercise 1.4 | Set 2

### Question 7: Is ∗ defined on the set {1, 2, 3, 4, 5} by a ∗ b = L.C.M. of a and b a binary operation? Justify your answer.

Solution:

The operation * on the set {1, 2, 3, 4, 5} is defined as

a * b = L.C.M. of a and b

Let a=3, b=5

3 * 5 = 5 * 3 = L.C.M. of 3 and 5 = 15 which does not belong to the given set

Thus, * is not a Binary Operation.

### Question 8: Let ∗ be the binary operation on N defined by a ∗ b = H.C.F. of a and b. Is ∗ commutative? Is ∗ associative? Does there exist identity for this binary operation on N?

Solution:

If a, b belongs to N

LHS = a * b = HCF of a and b

RHS = b * a = HCF of b and a

Since LHS = RHS

Therefore, * is Commutative

Now, If a, b, c belongs to Z, a * (b * c) = (a * b) * c

LHS = a * (b * c) = HCF of a, b and c

RHS = (a – b) * c = HCF of a, b and c

Since, LHS = RHS

Therefore, * is Associative

Now, 1 * a = a * 1 ≠ a

Thus, there doesn’t exist any identity element.

### Question 9: Let ∗ be a binary operation on the set Q of rational numbers as follows:

(i) a ∗ b = a – b

(ii) a ∗ b = a2 + b2

(iii) a ∗ b = a + ab

(iv) a ∗ b = (a – b)2

(v) a ∗ b = ab / 4

(vi) a ∗ b = ab2

### Find which of the binary operations are commutative and which are associative.

Solution:

(i) Commutative:

If a, b belongs to Z, a * b = b * a

LHS = a * b = a – b

RHS = b * a = b – a

Since, LHS is not equal to RHS

Therefore, * is not Commutative

Associative:

If a, b, c belongs to Z, a * (b * c) = (a * b) * c

LHS = a * (b * c) = a – (b – c) = a – b + c

RHS = (a – b) * c = a – b – c

Since, LHS is not equal to RHS

Therefore, * is not Associative

(ii) Commutative:

If a, b belongs to Z, a * b = b * a

LHS = a * b = a2 + b2

RHS = b * a = b2 + a2

Since, LHS is equal to RHS

Therefore, * is Commutative

Associative:

If a, b, c belongs to Z, a * (b * c) = (a * b) * c

LHS = a * (b * c) = a * (b2 + c2) = a2 + (b2 + c2)2

RHS = (a * b) * c = (a2 + b2) * c = (a2 + b2)2 + c2

Since, LHS is not equal to RHS

Therefore, * is not Associative

(iii) Commutative:

If a, b belongs to Z, a * b = b * a

LHS = a * b = a + ab

RHS = b * a = b + ba

Since, LHS is not equal to RHS

Therefore, * is not Commutative

Associative:

If a, b, c belongs to Z, a * (b * c) = (a * b) * c

LHS = a * (b * c) = a * (b + bc) = a + a(b + bc)

RHS = (a * b) * c = (a + ab) * c = a + ab + (a + ab)c

Since, LHS is not equal to RHS

Therefore, * is not Associative

(iv) Commutative:

If a, b belongs to Z, a * b = b * a

LHS = a * b = (a – b)2

RHS = b * a = (b – a)2

Since, LHS is not equal to RHS

Therefore, * is not Commutative

Associative:

If a, b, c belongs to Z, a * (b * c) = (a * b) * c

LHS = a * (b * c) = a * (b – c)2 = [a – (b – c)2]2

RHS = (a * b) * c = (a – b)2 * c = [(a – b)2  – c]2

Since, LHS is not equal to RHS

Therefore, * is not Associative

(v) Commutative:

If a, b belongs to Z, a * b = b * a

LHS = a * b = ab / 4

RHS = b * a = ba / 4

Since, LHS is equal to RHS

Therefore, * is Commutative

Associative:

If a, b, c belongs to Z, a * (b * c) = (a * b) * c

LHS = a * (b * c) = a * bc/4 = abc/16

RHS = (a * b) * c = ab/4 * c = abc/16

Since, LHS is equal to RHS

Therefore, * is Associative

(vi) Commutative:

If a, b belongs to Z, a * b = b * a

LHS = a * b = ab2

RHS = b * a = ba2

Since, LHS is not equal to RHS

Therefore, * is not Commutative

Associative:

If a, b, c belongs to Z, a * (b * c) = (a * b) * c

LHS = a * (b * c) = a * (bc)2 = a(bc2)2

RHS = (a * b) * c = (ab2) * c = ab2c2

Since, LHS is not equal to RHS

Therefore, * is not Associative

### Question 10: Find which of the operations given above has identity

Solution:

An element e ∈ Q will be the identity element for the operation * if

a * e = a = e * a, for a ∈ Q

for (v) a * b = ab/4

Let e be an identity element

a * e = a = e * a

LHS : ae/4 = a

=> e = 4

RHS : ea/4 = a

=> e = 4

LHS = RHS

Thus, Identity element exists

Other operations doesn’t satisfy the required conditions.

Hence, other operations doesn’t have identity.

### Show that ∗ is commutative and associative. Find the identity element for ∗ on A, if any.

Solution:

Given (a, b) * (c, d) = (a+c, b+d) on A

Let (a, b), (c, d), (e,f) be 3 pairs ∈ A

Commutative :

LHS = (a, b) * (c, d) = (a+c, b+d)

RHS = (c, d) * (a, b) = (c+a, d+b) = (a+c, b+d)

Since, LHS is equal to RHS

Therefore, * is Commutative

Associative:

If a, b, c belongs to Z, a * (b * c) = (a * b) * c

LHS = (a, b) * [(c, d) * (e, f)] = (a, b) * (c+e, d+f) = (a+c+e, b+d+f)

RHS = [(a, b) * (c, d)] * (e, f) = (a+c, b+d) * (e, f) = (a+c+e, b+d+f)

Since, LHS is equal to RHS

Therefore, * is Associative

Existence of Identity element:

For a, e ∈ A, a * e = a

(a, b) * (e, e) = (a, b)

(a+e, b+e) = (a, b)

a + e = a

=> e = 0

b + e = b

=> e = 0

As 0 is not a part of set of natural numbers. So, identity function does not exist.

### Question 12: State whether the following statements are true or false. Justify.

(i) For an arbitrary binary operation ∗ on a set N, a ∗ a = a ∀ a ∈ N.

(ii) If ∗ is a commutative binary operation on N, then a ∗ (b ∗ c) = (c ∗ b) ∗ a

Solution:

(i) Let * be an operation on N, defined as:

a * b =  a + b ∀ a, b ∈ N

Let us consider b = a = 6, we have:

6 * 6 = 6 + 6 = 12 ≠ 6

Therefore, this statement is false.

(ii) Since, * is commutative

LHS = a ∗ (b ∗ c) = a * (c * b) = (c * b) * a = RHS

Therefore, this statement is true.

### Question 13: Consider a binary operation ∗ on N defined as a ∗ b = a3+ b3. Choose the correct answer.

(A) Is ∗ both associative and commutative?

(B) Is ∗ commutative but not associative?

(C) Is ∗ associative but not commutative?

(D) Is ∗ neither commutative nor associative?

Solution:

On N, * is defined as a * b = a3 + b3

Commutative:

If a, b belongs to Z, a * b = b * a

LHS = a * b = a3 + b3

RHS = b * a = b3 + a3

Since, LHS is equal to RHS

Therefore, * is Commutative

Associative:

If a, b, c belongs to Z, a * (b * c) = (a * b) * c

LHS = a * (b * c) = a * (b3 + c3) = a3 + (b3 + c3)3

RHS = (a * b) * c = (a3 + b3) * c = (a3 + b3)3 + c3

Since, LHS is not equal to RHS

Therefore, * is not Associative

Thus, Option (B) is correct.

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