NCERT Solutions
Class 11 Maths
Sets

Ex.Misc.Q.8
Show that for any sets A and B,
A = (A ∩ B) ∪ (A – B) and A ∪ (B – A) = (A ∪ B)
To show: A = (A ∩ B) ∪ (A – B)
Let x ∈ A
We have to show that x ∈ (A ∩ B) ∪ (A – B)
Case 1: x ∈ A ∩ B
Then, x ∈ (A ∩ B) ⊂ (A ∪ B) ∪ (A – B)
Case 2: x ∉ A ∩ B
=> x ∉ A or x ∉ B
=> x ∉ B [x ∉ A]
=> x ∉ A – B ⊂ (A ∪ B) ∪ (A – B)
So, A ⊂ (A ∩ B) ∪ (A – B) .......... (1)
It is clear that
A ∩ B ⊂ A and (A – B) ⊂ A
So, (A ∩ B) ∪ (A – B) ⊂ A ........ (2)
From equation (1) and (2), we obtain
A = (A ∩ B) ∪ (A – B)
To prove: A ∪ (B – A) ⊂ A ∪ B
Let x ∈ A ∪ (B – A)
=> x ∈ A or x ∈ (B – A)
=> x ∈ A or (x ∈ B and x ∉ A)
=> (x ∈ A or x ∈ B) and (x ∈ A or x ∉ A)
=> x ∈ (A ∪ B)
So, A ∪ (B – A) ⊂ (A ∪ B) .......... (3)
Next, we show that (A ∪ B) ⊂ A ∪ (B – A).
Let y ∈ A ∪ B
=> y ∈ A or y ∈ B
=> (y ∈ A or y ∈ B) and (y ∈ A or y ∉ A)
=> y ∈ A or (y ∈ B and y ∉ A)
=> y ∈ A ∪ (B – A)
So, A ∪ B ⊂ A ∪ (B – A) .......... (4)
Hence, from equation (3) and (4), we get
A ∪ (B – A) = A ∪ B.