Expand :

(i) (x - 2y + 6)(x - 2y - 6).

(ii) (2a - 4b + 7) (2a + 4b + 7).

If a + b = 1 and a - b = 7; find:

(i) a² + b²

(ii) ab

If x - y + z = 5 and x² + y² + z² = 49, find the value of : zx - xy - yz.

Factorise :

x(a - 5) + y(5 - a).