x = y                   | * x
=>      x^2 = xy                | + (x^2 - 2xy)
=>      2x^2 - 2xy = x^2 - xy   |
=>      2(x^2 - xy) = x^2 - xy  | / (x^2 - xy)   --- WRONG.
=>      2 = 1

If x = y, then (x^2 - xy) = 0, and since nothing can be divided by zero, this cannot take place.