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This question already has an answer here:

Try to find the step where I made a mistake while proving that 1 = 2

  1. a = 1

  2. b = 1

  3. a = b

  4. a^2 = ab

  5. a^2 - b^2 = ab - b^2

  6. (a + b)(a - b) = b(a - b)

  7. a + b = b

  8. 1 + 1 = 1

  9. 2 = 1

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marked as duplicate by Alconja, Ankoganit, Jamal Senjaya, Deusovi Jan 10 '18 at 5:09

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It is not correct at step 6 to 7, because you divide both of the side by (a - b) which is dividing by 0.

Why can't you divide by zero?

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