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Puzzle ZIHG 



??

A Proof That 2 = 1. Let A = B:

           A = B

multiply by A on both sides:

         A x A = A x B

subtract B x B from both sides:

   A x A – B x B = A x B – B x B

factorise:

  (A + B)(A – B) = B(A – B)

divide by (A – B) on both sides:

       (A + B) = B

replace A with B:

         B + B = B

factorise:

          2B = B

divide by B on both sides:

           2 = 1

Where lies the flaw in the logic?

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Answer
On line 4 we are dividing both sides by (A – B).

But we know that A = B, so A – B = 0.

Dividing by zero is undefined, so we are not allowed to do this.

 

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