Hello guys, I'm having some trouble proving this (it's from the introduction of Apostol's calculus book). Thx

(a) Prove that one of the following two formulas is always right and the other one is sometimes wrong:

(i) $\displaystyle \text A - (B - C) = (A - B) \cup C$

(ii) $\displaystyle \text A - (B \cup C) = (A - B) - C$

(b) State an additional necessary and sufficient condition for the formula which is sometimes incorrect to be always right.