The question:

What is wrong with the following "proof"? Let x=y. Then

Solution: I know that 2 does not equal 1 and my answer was an incorrect substitution in step 5 however that makes no sense as x=y. The book says that in step 3 'they' incorrectly divided by (x-y)=0. I do not understand, if (x-y) is a whole number then would it not be valid to perform the division?

i.e. ?

Now, perhaps this post would be better suited in the set theory/logic section but as the concept under discussion is of elementary character I felt that would be an unnecessary aggrandizement. Also I am unsure as to the formal system of 'proof' being employed, is it deduction?