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# RD Sharma solutions for Class 12 Mathematics chapter 3 - Binary Operations

## Chapter 3 - Binary Operations

#### Pages 4 - 5

Q 1.1 | Page 4

Determine whether of the following operation define a binary operation on the given set or not :$' * ' \text{on N defined by a * b} = a^b \text{ for all a, b} \in N .$

Q 1.2 | Page 4

Determine whether of the following operation define a binary operation on the given set or not : $'O' \text{ on Z defined by a O b } = a^b \text{ for all a,} b \in Z .$

Q 1.3 | Page 4

Determine whether of the following operation define a binary operation on the given set or not : $' * ' \text{on N defined by a * b} = \text{ a + b - 2 for all a, b} \in N$.

Q 1.4 | Page 4

Determine whether of the following operation define a binary operation on the given set or not :$' \times_6 ' \text{on S} = \left\{ 1, 2, 3, 4, 5 \right\} \text{defined by}$

$a \times_6 b = \text{ Remainder when ab is divided by } 6 .$

Q 1.5 | Page 4

Determine whether of the following operation define a binary operation on the given set or not :

$' +_6 ' \text{on S} = \left\{ 0, 1, 2, 3, 4, 5 \right\} \text{defined by}$
$a +_6 b = \begin{cases}a + b & ,\text{ if a} + b < 6 \\ a + b - 6 & , \text{if a} + b \geq 6\end{cases}$

Q 1.6 | Page 4

Determine whether of the following operation define a binary operation on the given set or not :

$' \odot ' \text{on N defined by a} \odot b = a^b + b^a \text{ for all a, b} \in N$

Q 1.7 | Page 4

Determine whether of the following operation define a binary operation on the given set or not :

$' * ' \text{on Q defined by } a * b = \frac{a - 1}{b + 1} \text{for all a, b} \in Q .$

Q 2.1 | Page 4

Determine whether or not of the definition of * given below gives a binary operation. In the event that * is not a binary operation give justification of this.
On Z+, defined * by a * b = a − b

Here, Z+ denotes the set of all non-negative integers.

Q 2.2 | Page 4

Determine whether or not definition of *given below gives a binary operation. In the event that * is not a binary operation give justification of this.

On Z+, defined * by a * b = ab

Here, Z+ denotes the set of all non-negative integers.

Q 2.3 | Page 4

Determine whether or not of the definition of * given below gives a binary operation. In the event that * is not a binary operation give justification of this.

On R, define by a*b = ab2

Here, Z+ denotes the set of all non-negative integers.

Q 2.4 | Page 4

Determine whether or not of the definition of * given below gives a binary operation. In the event that * is not a binary operation give justification of this.

On Z+ define * by a * b = |a − b|

Here, Z+ denotes the set of all non-negative integers.

Q 2.5 | Page 4

Determine whether or not of the definition of * given below gives a binary operation. In the event that * is not a binary operation give justification of this.

On Z+, define * by a * b = a

Here, Z+ denotes the set of all non-negative integers.

Q 2.6 | Page 4

Determine whether or not of the definition of * given below gives a binary operation. In the event that * is not a binary operation give justification of this.

On R, define * by a * b = a + 4b2

Here, Z+ denotes the set of all non-negative integers.

Q 3 | Page 4

Let * be a binary operation on the set I of integers, defined by a * b = 2a + b − 3. Find the value of 3 * 4.

Q 4 | Page 4

Is * defined on the set {1, 2, 3, 4, 5} by a * b = LCM of a and b a binary operation? Justify your answer.

Q 5 | Page 5

Let S = {abc}. Find the total number of binary operations on S.

Q 6 | Page 5

Find the total number of binary operations on {ab}.

Q 7 | Page 5

Let S be the set of all rational numbers of the form $\frac{m}{n}$ , where m ∈ Z and n = 1, 2, 3. Prove that * on S defined by a * b = ab is not a binary operation.

Q 8 | Page 5

Prove that the operation * on the set

$M = \left\{ \begin{bmatrix}a & 0 \\ 0 & b\end{bmatrix}; a, b \in R - \left\{ 0 \right\} \right\}$ defined by A * B = AB is a binary operation.

Q 9 | Page 5

The binary operation * : R $\times$ R $\to$ R is defined as a * b = 2a + b. Find (2 * 3) * 4.  [CBSE 2012]

Q 10 | Page 5

Let * be a binary operation on N given by a * b = LCM (a, b) for all a, b$\in$ N. Find 5 * 7.   [CBSE 2012]

#### Pages 12 - 13

Q 1 | Page 12

Let '*' be a binary operation on N defined by
a * b = 1.c.m. (ab) for all ab ∈ N
(1) Find 2 * 4, 3 * 5, 1 * 6.
(2) Check the commutativity and associativity of '*' on N.

Q 2.1 | Page 12

Determine which of the following binary operation is associative and which is commutative : * on N defined by a * b = 1 for all a, b ∈ N ?

Q 2.2 | Page 12

Determine which of the following binary operations are associative and which are commutative : * on Q defined by $a * b = \frac{a + b}{2} \text{ for all a, b } \in Q$ ?

Q 3 | Page 12

Let A be any set containing more than one element. Let '*' be a binary operation on Adefined by a * b = b for all ab ∈ Is '*' commutative or associative on ?

Q 4.01 | Page 12

Check the commutativity and associativity of the following binary operation '*'. on Z defined by a * b = a + b + ab for all ab ∈ Z ?

Q 4.02 | Page 12

Check the commutativity and associativity of the following binary operations '*'. on N defined by a * b = 2ab for all ab ∈ N ?

Q 4.03 | Page 12

Check the commutativity and associativity of the following binary operations '*'. on Q defined by a * b = a − b for all a, b ∈ Q ?

Q 4.04 | Page 12

Check the commutativity and associativity of the following binary operations '⊙' on Q defined by a ⊙ b = a2 + b2 for all ab ∈ Q ?

Q 4.05 | Page 12

Check the commutativity and associativity of the following binary operation 'o' on Q defined by $a o b = \frac{ab}{2}$ for all a, b ∈ Q ?

Q 4.06 | Page 12

Check the commutativity and associativity of the following binary operation '*' on Q defined by a * b = ab2 for all ab ∈ Q ?

Q 4.07 | Page 12

Check the commutativity and associativity of the following binary operation '*' on Q defined by a * b = a + ab for all ab ∈ Q ?

Q 4.08 | Page 12

Check the commutativity and associativity of the following binary operation  '*' on R defined by a * b = a + b − 7 for all ab ∈ R ?

Q 4.09 | Page 12

Check the commutativity and associativity of the following binary operation'*' on Q defined by a * b = (a − b)2 for all ab ∈ Q ?

Q 4.1 | Page 12

Check the commutativity and associativity of the following binary operation'*' on Q defined by a * b = ab + 1 for all ab ∈ Q ?

Q 4.11 | Page 12

Check the commutativity and associativity of the following binary operation '*' on N, defined by a * b = ab for all ab ∈ N ?

Q 4.12 | Page 12

Check the commutativity and associativity of the following binary operation '*' on Z defined by a * b = a − b for all ab ∈ Z ?

Q 4.13 | Page 12

Check the commutativity and associativity of the following binary operation '*' on Q defined by $a * b = \frac{ab}{4}$ for all ab ∈ Q ?

Q 4.14 | Page 12

Check the commutativity and associativity of the following binary operation  '*' on Z defined by a * b = a + b − ab for all ab ∈ Z ?

Q 4.15 | Page 12

Check the commutativity and associativity of the following binary operation '*' on N defined by a * b = gcd(ab) for all ab ∈ N ?

Q 5 | Page 12

If the binary operation o is defined by aob = a + b − ab on the set Q − {−1} of all rational numbers other than 1, shown that o is commutative on Q − [1].

Q 6 | Page 12

Show that the binary operation * on Z defined by a * b = 3a + 7b is not commutative ?

Q 7 | Page 12

On the set Z of integers a binary operation * is defined by a * b = ab + 1 for all a , b ∈ Z. Prove that * is not associative on Z.

Q 8 | Page 12

Let S be the set of all real numbers except −1 and let '*' be an operation defined by a * b = a + b + ab for all ab ∈ S. Determine whether '*' is a binary operation on S. If yes, check its commutativity and associativity. Also, solve the equation (2 * x) * 3 = 7.

Q 9 | Page 12

On Q, the set of all rational numbers, * is defined by$a * b = \frac{a - b}{2}$ , shown that * is no associative ?

Q 10 | Page 12

On Z, the set of all integers, a binary operation * is defined by a * b = a + 3b − 4. Prove that * is neither commutative nor associative on Z.

Q 11 | Page 12

On the set Q of all ration numbers if a binary operation * is defined by $a * b = \frac{ab}{5}$ , prove that * is associative on Q.

Q 12 | Page 12

The binary operation * is defined by $a * b = \frac{ab}{7}$ on the set Q of all rational numbers. Show that * is associative.

Q 13 | Page 13

On Q, the set of all rational numbers a binary operation * is defined by $a * b = \frac{a + b}{2}$ Show that * is not associative on Q.

Q 14.1 | Page 13

Let S be the set of all rational numbers except 1 and * be defined on S by a * b = a + b $-$ ab, for all a, b $\in$ S:

Prove that * is a binary operation on S ?

Q 14.2 | Page 13

Let S be the set of all rational numbers except 1 and * be defined on S by a * b = a + b $-$ ab, for all a, b $\in$ S:

Prove that * is commutative as well as associative ?

#### Page 15

Q 1 | Page 15

Find the identity element in the set I+ of all positive integers defined by a * b = a + b for all ab ∈ I+.

Q 2 | Page 15

Find the identity element in the set of all rational numbers except −1 with respect to *defined by a * b = a + b + ab.

Q 3 | Page 15

If the binary operation * on the set Z is defined by a * b = a + b −5, the find the identity element with respect to *.

Q 4 | Page 15

On the set Z of integers, if the binary operation * is defined by a * b = a + b + 2, then find the identity element.

#### Page 25

Q 1.1 | Page 25

Let * be a binary operation on Z defined by
a * b = a + b − 4 for all a, b ∈ Z Show that '*' is both commutative and associative ?

Q 1.2 | Page 25

Let * be a binary operation on Z defined by
a * b = a + b − 4 for all a, b ∈ Z Find the identity element in Z ?

Q 1.3 | Page 25

Let * be a binary operation on Z defined by
a * b = a + b − 4 for all a, b ∈ Z Find the invertible elements in Z ?

Q 2 | Page 25

Let * be a binary operation on Q0 (set of non-zero rational numbers) defined by $a * b = \frac{ab}{5} \text{for all a, b} \in Q_0$

Show that * is commutative as well as associative. Also, find its identity element if it exists.

Q 3.1 | Page 25

(i) Let * be a binary operation on Q − {−1} defined by a * b = a + b + ab for all a, b ∈ Q − {−1} Show that '*' is both commutative and associative on Q − {−1}.

Q 3.2 | Page 25

Let * be a binary operation on Q − {−1} defined by a * b = a + b + ab for all a, b ∈ Q − {−1} Find the identity element in Q − {−1} ?

Q 3.3 | Page 25

Let * be a binary operation on Q − {−1} defined by a * b = a + b + ab for all a, b ∈ Q − {−1} Show that every element of Q − {−1} is invertible. Also, find the inverse of an arbitrary element ?

Q 4.1 | Page 25

Let A = R0 × R, where R0 denote the set of all non-zero real numbers. A binary operation '⊙' is defined on A as follows (a, b) ⊙ (c, d) = (ac, bc + d) for all (a, b), (c, d) ∈ R0 × R :

Show that '⊙' is commutative and associative on A ?

Q 4.2 | Page 25

Let A = R0 × R, where R0 denote the set of all non-zero real numbers. A binary operation '⊙' is defined on A as follows (ab) ⊙ (cd) = (acbc + d) for all (ab), (cd) ∈ R0 × R :

Find the identity element in A ?

Q 4.3 | Page 25

Let A = R0 × R, where R0 denote the set of all non-zero real numbers. A binary operation '⊙' is defined on A as follows (a, b) ⊙ (c, d) = (ac, bc + d) for all (a, b), (c, d) ∈ R0 × R :

Find the invertible elements in A ?

Q 5.1 | Page 25

Let 'o' be a binary operation on the set Q0 of all non-zero rational numbers defined by   $a o b = \frac{ab}{2}, \text{for all a, b} \in Q_0$.

Show that 'o' is both commutative and associate ?

Q 5.2 | Page 25

Let 'o' be a binary operation on the set Q0 of all non-zero rational numbers defined by $a o b = \frac{ab}{2}, \text{ for all a, b } \in Q_0$ :

Find the identity element in Q0.

Q 5.3 | Page 25

Let 'o' be a binary operation on the set Q0 of all non-zero rational numbers defined by  $a o b = \frac{ab}{2}, \text{ for all a, b } \in Q_0$:

Find the invertible elements of Q0 ?

Q 6 | Page 25

On R − {1}, a binary operation * is defined by a * b = a + b − ab. Prove that * is commutative and associative. Find the identity element for * on R − {1}. Also, prove that every element of R − {1} is invertible.

Q 7.1 | Page 25

Let R0 denote the set of all non-zero real numbers and let A = R0 × R0. If '*' is a binary operation on Adefined by (a, b) * (c, d) = (ac, bd) for all (a, b), (c, d) ∈ A :

Show that '*' is both commutative and associative on A ?

Q 7.2 | Page 25

Let R0 denote the set of all non-zero real numbers and let A = R0 × R0. If '*' is a binary operation on Adefined by (a, b) * (c, d) = (ac, bd) for all (a, b), (c, d) ∈ A:

Find the identity element in A ?

Q 7.3 | Page 25

Let R0 denote the set of all non-zero real numbers and let A = R0 × R0. If '*' is a binary operation on Adefined by (a, b) * (c, d) = (ac, bd) for all (a, b), (c, d) ∈ A:

Find the invertible element in A ?

Q 8 | Page 25

Let * be the binary operation on N defined by a * b = HCF of a and b.
Does there exist identity for this binary operation one N ?

Q 9 | Page 25

Let A  $=$ R  $\times$ R and $*$  be a binary operation on defined by $(a, b) * (c, d) = (a + c, b + d) .$ . Show that $*$ is commutative and associative. Find the binary element for $*$ on A, if any.

#### Pages 33 - 34

Q 1 | Page 33

Construct the composition table for ×4 on set S = {0, 1, 2, 3}.

Q 2 | Page 33

Construct the composition table for +5 on set S = {0, 1, 2, 3, 4}.

Q 3 | Page 33

Construct the composition table for ×6 on set S = {0, 1, 2, 3, 4, 5}.

Q 4 | Page 33

Construct the composition table for ×5 on Z5 = {0, 1, 2, 3, 4}.

Q 5 | Page 33

For the binary operation ×10 on set S = {1, 3, 7, 9}, find the inverse of 3.

Q 6 | Page 33

For the binary operation ×7 on the set S = {1, 2, 3, 4, 5, 6}, compute 3−1 ×7 4.

Q 7 | Page 33

Find the inverse of 5 under multiplication modulo 11 on Z11.

Q 8 | Page 33

Write the multiplication table for the set of integers modulo 5.

Q 9 | Page 33

Consider the binary operation * and o defined by the following tables on set S = {a, bcd}.

(1)

 * a b c d a a b c d b b a d c c c d a b d d c b a

(2)

 o a b c d a a a a a b a b c d c a c d b d a d b c

Show that both the binary operations are commutative and associatve. Write down the identities and list the inverse of elements.

Q 10 | Page 34

Define a binary operation * on the set {0, 1, 2, 3, 4, 5} as $a * b = \begin{cases}a + b & ,\text{ if a + b} < 6 \\ a + b - 6 & , \text{if a + b} \geq 6\end{cases}$

Show that 0 is the identity for this operation and each element a ≠ 0 of the set is invertible with 6 − a being the inverse of a.

#### Pages 35 - 36

Q 1 | Page 35

Write the identity element for the binary operation * on the set R0 of all non-zero real numbers by the rule $a * b = \frac{ab}{2}$ for all ab ∈ R0.

Q 2 | Page 35

On the set Z of all integers a binary operation * is defined by a * b = a + b + 2 for all ab ∈ Z. Write the inverse of 4.

Q 3 | Page 35

Define a binary operation on a set.

Q 4 | Page 35

Define a commutative binary operation on a set.

Q 5 | Page 35

Define an associative binary operation on a set.

Q 6 | Page 35

Write the total number of binary operations on a set consisting of two elements.

Q 7 | Page 35

Write the identity element for the binary operation * defined on the set R of all real numbers by the rule

$a * b = \frac{3ab}{7} \text{ for all a, b} \in R .$ ?

Q 8 | Page 35

Let * be a binary operation, on the set of all non-zero real numbers, given by $a * b = \frac{ab}{5} \text { for all a, b } \in R - \left\{ 0 \right\}$

Write the value of x given by 2 * (x * 5) = 10.

Q 9 | Page 35

Write the inverse of 5 under multiplication modulo 11 on the set {1, 2, ... ,10}.

Q 10 | Page 35

Define identity element for a binary operation defined on a set.

Q 11 | Page 36

Write the composition table for the binary operation multiplication modulo 10 (×10) on the set S = {2, 4, 6, 8}.

Q 12 | Page 36

For the binary operation multiplication modulo 10 (×10) defined on the set S = {1, 3, 7, 9}, write the inverse of 3.

Q 13 | Page 36

For the binary operation multiplication modulo 5 (×5) defined on the set S = {1, 2, 3, 4}. Write the value of $\left( 3 \times_5 4^{- 1} \right)^{- 1}.$

Q 14 | Page 36

Write the composition table for the binary operation ×5 (multiplication modulo 5) on the set S = {0, 1, 2, 3, 4}.

Q 15 | Page 36

A binary operation * is defined on the set R of all real numbers by the rule $a * b = \sqrt{ a^2 + b^2} \text{for all a, b } \in R .$

Write the identity element for * on R.

Q 16 | Page 36

Let +6 (addition modulo 6) be a binary operation on S = {0, 1, 2, 3, 4, 5}. Write the value of $2 +_6 4^{- 1} +_6 3^{- 1} .$

Q 17 | Page 36

Let * be a binary operation defined by a * b = 3a + 4b − 2. Find 4 * 5.

Q 18 | Page 36

If the binary operation * on the set Z of integers is defined by a * b = a + 3b2, find the value of 2 * 4.

Q 19 | Page 36

Let * be a binary operation on N given by a * b = HCF (a, b), a, b ∈ N. Write the value of 22 * 4.

Q 20 | Page 36

Let * be a binary operation on set of integers I, defined by a * b = 2a + b − 3. Find the value of 3 * 4.

#### Pages 36 - 39

Q 1 | Page 36

If a * b = a2 + b2, then the value of (4 * 5) * 3 is
(a) (42 + 52) + 32
(b) (4 + 5)2 + 32
(c) 412 + 32
(d) (4 + 5 + 3)2

Q 2 | Page 36

If a * b denote the bigger among a and b and if a ⋅ b = (a * b) + 3, then 4.7 =
(a) 14
(b) 31
(c) 10
(d) 8

Q 3 | Page 37

On the power set P of a non-empty set A, we define an operation ∆ by

$X ∆ Y = \left( X \cap Y \right) \cup \left( X \cap Y \right)$

Then which are of the following statements is true about ∆
(a) commutative and associative without an identity
(b) commutative but not associative with an identity
(c) associative but not commutative without an identity
(d) associative and commutative with an identity

Q 4 | Page 37

If the binary operation * on Z is defined by a * b = a2 − b2 + ab + 4, then value of (2 * 3) * 4 is
(a) 233
(b) 33
(c) 55
(d) −55

Q 5 | Page 37

Mark the correct alternative in the following question:
For the binary operation * on Z defined by a * b = a + b + 1, the identity clement is

(a) 0

(b)$-$ 1

(c) 1

(d) 2

Q 6 | Page 37

If a binary operation * is defined on the set Z of integers as a * b = 3a − b, then the value of (2 * 3) * 4 is
(a) 2
(b) 3
(c) 4
(d) 5

Q 7 | Page 37

Q+ denote the set of all positive rational numbers. If the binary operation a ⊙ on Q+ is defined as $a \odot = \frac{ab}{2}$ ,then the inverse of 3 is

(a) $\frac{4}{3}$

(b) 2

(c) $\frac{1}{3}$

(d) $\frac{2}{3}$

Q 8 | Page 37

If G is the set of all matrices of the form

$\begin{bmatrix}x & x \\ x & x\end{bmatrix}, \text{where x } \in R - \left\{ 0 \right\}$ then the identity element with respect to the multiplication of matrices as binary operation, is

(a) $\begin{bmatrix}1 & 1 \\ 1 & 1\end{bmatrix}$

(b) $\begin{bmatrix}- 1/2 & - 1/2 \\ - 1/2 & - 1/2\end{bmatrix}$

(c) $\begin{bmatrix}1/2 & 1/1 \\ 1/2 & 1/2\end{bmatrix}$

(d)  $\begin{bmatrix}- 1 & - 1 \\ - 1 & - 1\end{bmatrix}$

Q 9 | Page 37

Q+ is the set of all positive rational numbers with the binary operation * defined by $a * b = \frac{ab}{2}$ for all ab ∈ Q+. The inverse of an element a ∈ Q+ is
(a) a
(b) $\frac{1}{a}$

(c) $\frac{2}{a}$

(d) $\frac{4}{a}$

Q 10 | Page 37

If the binary operation ⊙ is defined on the set Q+ of all positive rational numbers by $a \odot b = \frac{ab}{4} . \text{ Then }, 3 \odot \left( \frac{1}{5} \odot \frac{1}{2} \right)$ is equal to
(a) $\frac{3}{160}$

(b) $\frac{5}{160}$

(c) $\frac{3}{10}$

(d) $\frac{3}{40}$

Q 11 | Page 37

Let * be a binary operation defined on set Q − {1} by the rule a * b = a + b −ab. Then, the identify element for * is
(a) 1
(b) $\frac{a - 1}{a}$

(c) $\frac{a}{a - 1}$

(d)  0

Q 12.1 | Page 37

Which of the following is true?
* defined by $a * b = \frac{a + b}{2}$ is a binary operation on Z .

Q 12.2 | Page 37

Which of the following is true?

* defined by $a * b = \frac{a + b}{2}$  is a binary operation on Q .

Q 12.3 | Page 37

Which of the following is true?

all binary commutative operations are associative

Q 12.4 | Page 37

Which of the following is true?

subtraction is a binary operation on N

Q 13 | Page 38

The binary operation * defined on N by
a * b = a + b + ab for all ab ∈ N is
(a) commutative only
(b) associative only
(c) commutative and associative both
(d) none of these

Q 14 | Page 38

The binary operation * is defined by a * b = a2 + b2 + ab + 1, then (2 * 3) * 2 is equal to
(a) 20
(b) 40
(c) 400
(d) 445

Q 15 | Page 38

Let * be a binary operation on R defined by a * b = ab + 1. Then, * is
(a) commutative but not associative
(b) associative but not commutative
(c) neither commutative nor associative
(d) both commutative and associative

Q 16 | Page 38

Subtraction of integers is
(a) commutative but no associative
(b) commutative and associative
(c) associative but not commutative
(d) neither commutative nor associative

Q 17 | Page 38

The law a + b = b + a is called
(a) closure law
(b) associative law
(c) commutative law
(d) distributive law

Q 18 | Page 38

An operation * is defined on the set Z of non-zero integers by $a * b = \frac{a}{b}$  for all ab ∈ Z. Then the property satisfied is

(a) closure
(b) commutative
(c) associative
(d) none of these

Q 19 | Page 38

On Z an operation * is defined by a * b = a2 + b2 for all a, b ∈ Z. The operation * on Z is
(a) commutative and associative
(b) associative but not commutative
(c) not associative
(d) not a binary operation

Q 20 | Page 38

A binary operation * on Z defined by a * b = 3a + b for all a, b ∈ Z, is
(a) commutative
(b) associative
(c) not commutative
(d) commutative and associative

Q 21 | Page 38

Let * be a binary operation on Q+ defined by $a * b = \frac{ab}{100} \text{ for all a, b } \in Q^+$ The inverse of 0.1 is

(a) 105
(b) 104
(c) 106
(d) none of these

Q 22 | Page 38

Let * be a binary operation on N defined by a * b = a + b + 10 for all ab ∈ N. The identity element for * in N is
(a) −10
(b) 0
(c) 10
(d) non-existent

Q 23 | Page 38

Consider the binary operation * defined on Q − {1} by the rule
a * b = a + b − ab for all a, b ∈ Q − {1}
The identity element in Q − {1} is
(a) 0
(b) 1
(c) $\frac{1}{2}$

(d) −1

Q 24 | Page 38

For the binary operation * defined on R − {1} by the rule a * b = a + b + ab for all a, b ∈ R − {1}, the inverse of a is

(a) $- a$

(b) $- \frac{a}{a + 1}$

(c) $\frac{1}{a}$

(d) $a^2$

Q 25 | Page 38

For the multiplication of matrices as a binary operation on the set of all matrices of the form

$\begin{bmatrix}a & b \\ - b & a\end{bmatrix}$ a, b ∈ R the inverse of

$\begin{bmatrix}2 & 3 \\ - 3 & 2\end{bmatrix}$ is

(a) $\begin{bmatrix}- 2 & 3 \\ - 3 & - 2\end{bmatrix}$

(b) $\begin{bmatrix}2 & 3 \\ - 3 & 2\end{bmatrix}$

(c)  $\begin{bmatrix}2/13 & - 3/13 \\ 3/13 & 2/13\end{bmatrix}$

(d) $\begin{bmatrix}1 & 0 \\ 0 & 1\end{bmatrix}$

Q 26 | Page 38

On the set Q+ of all positive rational numbers a binary operation * is defined by

$a * b = \frac{ab}{2} \text{ for all, a, b }\in Q^+$. The inverse of 8 is

(a) $\frac{1}{8}$

(b) $\frac{1}{2}$

(c) 2

(d) 4

Q 27 | Page 39

Let * be a binary operation defined on Q+ by the rule

$a * b = \frac{ab}{3} \text{ for all a, b } \in Q^+$ The inverse of 4 * 6 is

(a) $\frac{9}{8}$

(b) $\frac{2}{3}$

(c) $\frac{3}{2}$

(d) none of these

Q 28 | Page 39

The number of binary operation that can be defined on a set of 2 elements is
(a) 8
(b) 4
(c) 16
(d) 64

Q 29 | Page 39

The number of commutative binary operations that can be defined on a set of 2 elements is
(a) 8
(b) 6
(c) 4
(d) 2

## RD Sharma solutions for Class 12 Mathematics chapter 3 - Binary Operations

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Concepts covered in Class 12 Mathematics chapter 3 Binary Operations are Types of Relations, Types of Functions, Composition of Functions and Invertible Function, Inverse of a Function, Concept of Binary Operations, Introduction of Relations and Functions.

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