+33661 This is similar to my comment here: http://web2.0calc.com/questions/what-happens-when-you-divide-by-zero_2
The division by x - x is not allowed because x - x is zero. Division by zero is not allowed precisely because it leads to inconsistencies like 2 = 1 as you have demonstrated.
(For those who are puzzled by the move from the first to the second line: x2 - x2 can be written as x(x - x) because x is a common factor; and it can also be written as (x - x)(x + x) because it's the difference of two squares).
