1.ii
x^2 - y^2 = (x - y)(x + y) // P9
          = x(x + y) - y(x + y) // P9
          = x^2 + xy - xy - y^2
          = x^2 - y^2

1.iii
ir x^2 = y^2 then
x^2 - y^2 = 0
(x + y)(x - y) = 0
now there are two ways
x + y = 0 => x = -y
or
x - y = 0 => x = y

1.iv
same as 1.ii

1.v
?

1.vi
same as 1.ii

2.
division by zero in the 3rd step