21.
(i) 2 cos x cos y = cos (x + y) + cos (x - y)
(ii) - 2 sin x sin y = cos (x + y) - cos (x - y)
(iii) 2 sin x cos y = sin (x + y) + sin (x - y)
(iv) 2 cos x sin y = sin (x + y) - sin (x - y)
These are basic formula's of trigonometry. Basically you dont need know there derivations, you just need to remember them and implement them when you are doing sums. Just learn them and start with your exercises, things will be clear as crystal.