Find all solutions of the given equation:

1.) 2 sin x + 1 = 0

2.) sin 2x - cos x = 0

3.) sin² x - sin x = 0

**Not looking for the entire answer, just how to start.

Prove that the given trig identity is true:

cos (a - b) = (cos a) (cos b) + (sin a) (sin b)

**I know that it's true because it states the formula in the book but I just don't know how to prove it, lol.