I got help with one proof already, I am just lost when it comes to proofs....here is another on...

Problem:

Prove that the empty set is unique. That is, suppose that A and B are empty sets and prove that A=B

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- February 1st 2007, 07:44 PMluckyc1423Proof of sets
I got help with one proof already, I am just lost when it comes to proofs....here is another on...

Problem:

Prove that the empty set is unique. That is, suppose that A and B are empty sets and prove that A=B - February 1st 2007, 07:58 PMThePerfectHacker
If we are working in the Zermelo-Frankael Set theory model, we will use the axiom of extensionality.

"Two sets are equal if and only if for any element of A if and only if is an element of B".

Since A has no elements the statement,

"If x in A then x in B" is true.

Similarly,

"If x in B then x in A" is true.

These statements are true because the hypothesis of the conditional is false. And a false impling a true of a false impling a true is always true. Thus these statements are both true. - February 3rd 2007, 05:29 AMSoroban
Hello, luckyc1423!

Quote:

Prove that the empty set is unique.

That is, suppose that and are empty sets. .Prove that .

Let and

When are two sets and**un**equal?

They are unequal if there is an element in which is not in

. . . . . . . . . . .or if there is an element in which is not in .

(1) Is there an element in which is not in ?

. . . Since , the answer is No.

(2) Is there an element in whichis ot in ?

. . . Since , the answer is No.

Therefore: .