Puzzle 66 - 2 =1 ????

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 This puzzle is from the book by world famous Shakuntala Devi

What is wrong with this proof ?

a=b
multiply both the sides by a
a2 = ab 
add -b2 to both the side
a2-b2 =ab-b2 
factorizing
(a+b)(a-b) = b(a-b) 
divide both the sides by (a-b)
a+b=b
Substituting a with b,
2b = b 
2=1

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