1.

For any two sets A and B, show that A × B and B × A have an element in common if and only if A and B have an element in common.

Answer»

We know, 

(A × B) ∩ (B × A) = (A ∩ B) × (B ∩ A) 

Here A and B have an element in common i.e., n(A ∩ B) = 1 = (B ∩ A) 

So, n((A × B) ∩ (B × A)) = n((A ∩ B) × (B ∩ A)) = n(A ∩ B) × n(B ∩ A) = 1 × 1 = 1 

That means, A × B and B × A have an element in common if and only if A and B have an element in common. [Proved] 



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