- Prove some fundamentals laws of algebra of sets :
- A U A=A
- A intersection A=A
- Identity laws for any set A
- Commutative laws for any sets A & B
- Associative laws, A,B&C sets
- DISTRIBUTIVE LAWS, A,B&C SETS
- DE MORGANS LAW, for any sets A & B
(1) A ∪ A = A
Let A = {x : x ∈ A}
A ∪ A = {x : x ∈ a or x ∈ A}
= {x : x ∈ A}
= A
(2) Identity law, A ∪ Ø = A
Let A = {x : x ∈ A}
A ∪ Ø = {x : x ∈ A or x ∈ Ø}
= {x : x ∈ A}
= A
(3) Commutative law, A ∪ B = B ∪ A
Let x ∈ A ∪ B
Then, x ∈ A or x ∈ B
⇒ x ∈ B or x ∈ A
⇒ x ∈ B ∪ A
⇒ A ∪ B ⊆ B ∪ A
Similarly, B ∪ A ⊆ A ∪ B
⇒ A ∪ B = B ∪ A
(4) De-morgan's law, (A ∪ B)' = A' ∩ B'
Let x ∈ (A ∪ B)'
⇒ x (A ∪ B)
⇒ x A and x B
⇒ x ∈ A' and x ∈ B'
⇒ x ∈ A' ∩ B'
⇒ (A ∪ B)' ⊆ A' ∩ B'
Now, let y ∈ A' ∩ B'
⇒ y ∈ A' and y ∈ B'
⇒ y A and y B
⇒ y A ∪ B
⇒ y ∈ (A ∪ B)'
⇒ A' ∩ B' ⊆ (A ∪ B)'
Hence,(A ∪ B)' = A' ∩ B'
Similarly, you can prove other laws also.