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Class 12 RD Sharma Solutions- Chapter 3 Binary Operations – Exercise 3.3

  • Last Updated : 19 Jan, 2021

Question 1. Find the identity element in the set I+ of all positive integers defined by a * b = a + b for all a, b ∈ I+.

Solution:

Let e be the identity element in I+ with respect to * such that

a * e = a = e * a, ∀ a ∈ I+

a * e = a and e * a = a, ∀ a ∈ I+

a + e = a and e + a = a, ∀ a ∈ I+

e = 0, ∀ a ∈ I+

Hence, 0 is the identity element in I+ with respect to *.

Question 2. Find the identity element in the set of all rational numbers except – 1 with respect to * defined by a * b = a + b + ab

Solution:

Let e be the identity element in I+ with respect to * such that

a * e = a = e * a, ∀ a ∈ Q – {-1}

a * e = a and e * a = a, ∀ a ∈ Q – {-1}

a + e + ae = a and e + a + ea = a, ∀ a ∈ Q – {-1}

e + ae = 0 and e + ea = 0, ∀ a ∈ Q – {-1}

e (1 + a) = 0 and e (1 + a) = 0, ∀ a ∈ Q – {-1}

e = 0, ∀ a ∈ Q – {-1} [because a not equal to -1]

Hence, 0 is the identity element in Q – {-1} with respect to *.

Question 3. If the binary operation * on the set Z is defined by a*b = a + b – 5, then find the identity element with respect to *.

Solution: 

We are given the binary operator * defined on Z as

a*b = a + b – 5 for all a, b ∈ Q

Let e be the identity elements with respect to *

Then, a*e = e*a = a  [By identity property]

⇒ a + e – 5 = a

⇒ e = 5

Therefore, the required identity element with respect to * is 5.

Question 4. On the set Z integers, if the binary operation * is defined by a*b = a + b + 2, then find the identity elements.

Solution: 

The binary operator * is defined on Z, and is given by

a*b = a + b +2 for all a, b ∈ Z.

Let a ∈ Z and e ∈ Z be the identity element with respect to *, then

a*e = e*a = a    [By identity property]

⇒ a + e + 2 = a

⇒ e = -2 ∈ Z

Therefore, the identity element with respect to * is -2.

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