Question

Disprove this proof of the statement 2=1
a=b
a^2=ab
a^2-b^2=ab-b^2
(a-b)(a+b)=b(a-b)
((a-b)(a+b))/(a-b)=(b(a-b))/(a-b)
a+b=b
b+b=b
2b=b
2=1

Answer 1

when you divide with (a-b) in the fifth line, you must know that it's only possible when a is not equal to b, otherwise you will get dividing by zero which is not possible....so you can't divide by a-b because your condition in the first line is a=b

Answer 2

nice catch! :D
I gave this to my dad (a "Permanent-head-damage") and he laughed when he saw it.