1.

Prove that A × B = B × A ⇒ A = B.

Answer»

Let A and B be any two sets such that 

A × B = {(a, b): a ϵ A, b ϵ B} 

Now, 

B × A = {(b, a): a ϵ A, b ϵ B}

A × B = B × A 

(a, b) = (b, a) 

We can see that this is possible only when the ordered pairs are equal. 

Therefore, 

a = b and b = a 

Hence, Proved



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