x = y.
Then x^2 = xy.
Subtract the same thing from both sides:
x^2 - y^2 = xy - y^2.
Dividing by (x-y), obtain
x + y = y.
Since x = y, we see that
2 y = y.
Thus 2 = 1, since we started with y nonzero.
Subtracting 1 from both sides,
1 = 0.
Then x^2 = xy.
Subtract the same thing from both sides:
x^2 - y^2 = xy - y^2.
Dividing by (x-y), obtain
x + y = y.
Since x = y, we see that
2 y = y.
Thus 2 = 1, since we started with y nonzero.
Subtracting 1 from both sides,
1 = 0.