Use the formula: (a + b) (a - b) = a² - b² to evaluate:

(i) 21 x 19

(ii) 33 x 27

(iii) 103 x 97

(iv) 9.8 x 10.2

(v) 7.7 x 8.3

Evaluate:

(6 - xy) (6 + xy)