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Possible Duplicate:
Finding the error in a proof

  1. $a=b$
  2. $ab=a^2$
  3. $ab-b^2=a^2-b^2$
  4. $b(a-b)=(a+b)(a-b)$
  5. $b= a+b$

Reminder the first step where $b = 2b$ So,

$1=2$

In this case in my opinion the wrong step is that in the third, because you can´t subtract $b^2$ from both sides.

This question it was taken from a math exam.

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    I wonder why the most common questions can't be researched on [a search engine] before being asked.2012-10-21

3 Answers 3